If he gives his second wife the cost of tuition fees, is he obliged to give the same amount to his first wife?

Praise be to Allah.

Islam has made it incumbent upon the man who is married to more than one wife to treat his wives equitably in terms of spending on their maintenance. Equity in this case means spending on them according to his means, and giving each of them what she needs of maintenance and clothing, that is usually given to women like her. Based on that, one of them may need things that the other does not need in terms of maintenance. One who is studying may need, because of her study, things that the one who is not studying does not need. One who is pregnant may need maintenance that one who is not pregnant does not need. One who is sick may need things that the one who is not sick does not need, and so on. 

The point is that he is obliged to treat them equitably in terms of maintenance, so as to suffice each one of them. With regard to anything more than what is sufficient, he is not obliged to treat them equitably in that regard. 

Ibn Qudaamah said: 

He does not have to treat his wives equally in terms of maintenance and clothing if he has done what is obligatory in the case of each one of them. 

Ahmad said – concerning a man who had two wives: He may favour one of them over the other in terms of maintenance, intimacy and clothing, so long as the other one is provided for sufficiently. He may buy a garment for one that is more expensive, so long as the other one is adequately clothed. 

That is because equal treatment in all these matters is difficult, and if it was made obligatory, he would not be able to do it except with great difficulty. Thus this obligation is waived, as in the case of equal treatment in terms of intimacy.

End quote from al-Mughni, 7/232 

Al-Haafiz Ibn Hajar said: 

If he has fulfilled his obligations towards each one of them in terms of clothing, maintenance and spending the night, then it does not matter if he inclines more towards one of them or gives a gift to one of them.

End quote from Fath al-Baari, 9/391 

For more information, please see question no. 10091

Based on the above, what you must do is treat them equitably by spending on each one of them that which is sufficient for her. You do not have to give the first wife what you spent on the mahr and then on clothing and gold for the second one. 

And Allah knows best.