Is it permissible to combine two wives in one apartment? Is it permissible for the wife who has no children to help the other with her children's service? And is it permissible for her to refuse?

An independent dwelling is a right of the wife, and she is not obliged to live with her co-wife unless she consents. If the apartment is spacious enough for each wife to have her own facilities independently, in a way that prevents harm, then it is permissible to house both wives in it, even if they do not consent. However, combining them in one dwelling without independence is not permissible unless they both consent.

It is permissible for the husband to ask his wife to help her co-wife, and it is preferable for the wife to do so if it does not involve hardship or embarrassment, in order to increase affection. However, she is not obliged to obey him in this, because obedience is in what is right (ma'ruf), and this is not part of it.