What is the ruling on a divorce that takes place under duress from the first wife, and is the husband bound by the promises he made to himself?

The phrase written by the husband is an كناية (allusion/metaphorical expression) for divorce. If he intended divorce by it, then it occurs; otherwise, it does not. The husband must fulfill his promise and be just between his two wives in dividing time, unless one of them waives her right. The pressure from the first wife is not a coercion that prevents the occurrence of divorce. The acceptable coercion is by threatening with severe harm such as killing or beating.