How are apartments to be divided equitably between two brothers during their parents' lifetime, knowing that one of the two apartments they live in is of higher value, and that one of them previously received additional financial assistance?

There is no harm in dividing the apartments among the children. The value of the apartments at the time of division should be taken into account.

As for preferring one of the children with a gift, it is permissible if there is a legitimate reason, such as his need or a large family. In such a case, it is not necessary to equalize the others with him. However, if the preference is without a legitimate reason, it is not permissible, and they must give the other son the same as they gave his brother, unless he is content with that.