Is the husband obligated to be just between his two wives, despite his prior agreement with the first wife to allocate one day a week for her, and how is justice achieved?

If the husband stipulates to his first wife that he will only come to her once a week, or stipulates to the second that he will prefer her over others, then both conditions are void, but the marriage contract is valid. The wife has the right to demand her full rights in overnight stay (mabeet) and maintenance (nafaqa). The husband must deal justly between his wives, or divorce the one who demands her right. She may also waive her right willingly.