If you are sure that each of several marja’s are experts and more learned than others in a specific field, or at least equal in knowledge and understanding, then it is okay to follow more than one marja’.
We must say that you have first assumed that the basis of your question (which says that it isn’t permissible to follow more than marja’) is correct and then have asked about the reason behind it. In contrast to what has been assumed in your question, many faqihs or fiqh experts believe that in certain cases, not only is it permissible, but it is wajib to follow more than one marja’. Therefore, we will first list some of the fatwas on this issue and the Islamic reasoning behind it.
Imam Khomeini’s judgment: If two mujtahids are equal in knowledge (and none are more learned than the other), others (who aren’t mujtahids and have to practice taqlid) can refer to any of the two that they desire, and they can also follow one of the two in some issues and the other in other issues.
The Grand Ayatullah Khamenei also says: Not only isn’t tab’iz (following a mujtahid in some issues and another in other ones) problematic, but if there are marja’s that are each more learned than others in certain subjects of fiqh, it is necessary and wajib to follow each in the field that he is an expert in. Another viewpoint is that if there are two mujtahids in which one is an expert in worship while the other is an expert in transactions, it is necessary as a precaution to follow both of them. It has also been said that if two marja’s are equal in knowledge, one can follow any of the two that he/she chooses, and also if desired, he/she can follow one in some issues and the other in other issues. Therefore, what you stated (that one can only follow one marja’) isn’t accepted by many scholars. The only important point that must be explained here is the difference between udul (changing one’s marja’ after following a certain one) and tab’iz.
In order to explain the abovementioned points further, a few points must be noted:
1- The reason behind why every individual has to follow a particular marja’ has been explained as follows:
a) We are sure that every action that we do has an Islamic ruling that has been expressed by the imams, let it be a specific ruling on the action, or a general one that the act goes under.
b) We also know that Islamic scholars and narrators of hadiths have been assigned by the imams as references of Islamic rulings and issues.
c) Our hadiths haven’t assigned a particular person as the authority and official reference during the major occultation of Imam Zaman (as), what they do mention though are the characteristics of such scholars and references.
d) We can't follow all Islamic scholars, because they all don’t have the same viewpoints on various issues. We also can't follow an undistinguished scholar without knowing which one we are following, just like we can't be free and not follow anyone at all and do anything we like, because that is something that the mind won't accept and is also in contrast with what Muslims have practiced throughout history.
Hence, taking into consideration the characteristics mentioned in different hadiths, it is our duty to choose between the different individuals that meet the standards required for one to be followed, making him our marja’ taqlid.
e) After choosing the appropriate marja’ and following him in an issue, we can no longer follow another marja’ regarding that same issue. In other words, it isn’t possible to have two different taqlids regarding the same issue, unless the second marja’ has the same judgment as the first, or if one finds out that the first marja’ wasn’t qualified for being marja’ in the first place.
2- The difference between tab’iz and udul:
a) Tab’iz has two instances; the first being for one to follow two or more marja’s, each being an expert in a certain field, in the field they have an expertise in, and the second being for one to follow two or more marja’s that are completely equal in all fields, each in a certain field. Of course, both of these methods of tab’iz (which are both correct), apply to cases that the individual wants to begin taqlid for the first time.
b) Udul means for one to switch to a marja’ in a certain issue (or issues) although he/she has acted according to the judgment of another marja’ on that issue. This case differs from tab’iz and most scholars don’t see this case as permissible. Imam Khomeini says: There are only two ways one can switch between two live marja’s; the first being for the second marja’ to be equal and as learned as the first, which in this case it is permissible to switch to the second (udul). The second way is for the second marja’ to be more learned than the first, making it wajib to switch to the second.
Naturally, before becoming sure that the second is more learned or at least equal to the first, one cannot switch between marja’s without an acceptable reason.
Some scholars have explained the abovementioned ruling by saying that before following a marja’ in a specific ruling, it is okay to switch between marja’s (meaning that even if one has chosen a marja’ to follow, but still hasn’t followed him by acting according to his judgment, it is okay to change the marja’), but after acting according to a marja’s verdict, it is no longer permissible to change marja’s. For further information, on this matter, you can refer to Question 1335 of this website.
3- Another point that needs to be mentioned, is that in cases that one’s marja’ (that one has chosen because of him being the most learned) doesn’t have a definite fatwa on a particular issue and has issued an obligatory precaution instead (obligatory precaution refers to when a marja’ doesn’t reach a specific conclusion on a certain issue or has reached a conclusion but there are other reasons that don’t allow him to announce his viewpoint, he issues an obligatory precaution), one can refer to his/her next marja’ (which is the most learned after the first marja’) and act according to his fatwa if he has one and if not, he can refer to the next most learned and so on until a marja’ who has a definite fatwa instead of an obligatory precaution, or he/she can act according to his/her first marja’s obligatory precaution instead of referring to anyone else. Therefore, in short, in the case of one’s marja’ issuing an obligatory precaution on an issue, he/she can do one of two things; either act according to his obligatory precaution or refer to the next marja’ and follow his fatwa. This method of referring to another marja’ when one’s own marja’ issues a precaution can also somewhat be considered a form of tab’iz.
4- The final necessary note is that the believers shouldn’t make the subject of following more than one marja’ an excuse to follow any marja’ they desire in any issue just because his judgment on that issue goes with what they like. For instance, if someone is a smoker, and his marja’ says that smoking doesn’t break one’s fast, there is no problem for that individual to smoke while fasting. But if that same person’s marja’ says that smoking breaks one’s fast, and he goes on to follow another marja’ that believes that it is okay to smoke while fasting, without observing the necessary criteria for being able to switch between marja's, it isn’t acceptable and in reality this act isn’t called following a marja’ or taqlid, it is merely a way of justifying one’s acts that he/she desires.
 Seyyid Ruhullah Khomeini, Tahrirul-Wasilah, Darul-Ilm Institute, Qum, second print, vol. 1, pg. 6, issue 8.
 Seyyid Ali Khamenei, Ajwebatul-Istifta’at, Darul-Naba’ lil-Nashri wal-Tawzi’, first print, 1420 (ah), vol. 1, pg. 10, question 17.
 Seyyid Kadhim Yazdi, Al-Urwatul-Wuthqa, al-Nashrul-Islami Institute, 1404 (ah), first print, pg. 38, issue 47.
 Seyyid Muhsin Hakim, Mustamsakul-Urwatil-Wuthqa, Library of al-Seyyidul-Mar’ashi, 1404 (ah), vol.1, pg.?.
 Muhammad ibnil-Hasan Hurr al-Ameli, Wasa’elul-Shia, Aalul-Bayt Institute, Qum, 1409 (ah), vol. 27, pg. 140, hadith 33424.
 This reasoning is a summary of the late Ayatullah Seyyid Muhsin Hakim’s sayings in pp. 13 and 14 of the first volume of his book Mustamsakul-Urwah.
 Tahrirul-Wasilah, vol. 1, pg. 6, issue 4.
 Ibid, vol. 1, pg. 11, issue 34.
 There are many examples like smoking and here smoking was only mentioned as an example.