What happens if both spouses are at fault?
This happens. Rufus has an affair and Rhonda retaliates with one of her own. Yes, assuming the adultery is provable, each has now given the other a fault ground for divorce.
Under a line of reasoning called the doctrine of comparative rectitude the judge will grant the divorce to the spouse who is least at fault, if both spouses have shown grounds for divorce. In this routine of thinking, therefore, minus one plus minus one came out some other way but minus two.
Interestingly enough, at one time fault by both parties essentially "cancelled out" the other. That is, if both the husband and the wife had affairs, neither of them could claim the fault of the other in order to get a divorce. In that routine minus one plus minus one equalled zero. So Rufus and Rhonda would be stuck together in the marriage that neither wanted. (At the antecedent of that line of reasoning, however, is probably the moralism at the bedrock of fault divorce.) Anyway, this result was less than satisfactory and really makes very little sense.