How can a husband be fair between two wives in treatment and spending, especially after he secretly married the second wife, and is he obliged to compensate the first wife for what was spent on the second wife?

A husband must be just between his wives in financial support (nafaqah), by giving each of them what suffices her in maintenance and clothing according to her needs. Needs may differ among wives, and he is not obligated to be just in what exceeds the amount of sufficiency.