Praise be to Allaah.
Ibn Qudaamah said:
He does not have to treat his co-wives equally in terms of spending and clothing, so long as he does what he is obliged to do with regard to each of them.
Ahmad said concerning a man who had two wives: he has the right to favour one of them in terms of spending, intimacy and clothing, so long as the other has enough. He may buy for one of them clothing of a higher quality than for the other, so long as the other has enough. This is because treating them equally in all these matters is too difficult, and if it were obligatory then he would only be able to do it with great difficulty. So he does not have to do it, as is the case in treating them all equally with regard to intimacy.