Peter,
Finding somewhere to put masts in a cat is the determining factor, I think. In a cat of the "hulls and lashed beams" type, side by side is the only option, but clearly makes passage along the hulls difficult. In a bridgedeck cat, a single mast can be used, but two masts would rarely be possible.
In all two masted rigs it's fair to say that one sail is adversely affected by the other, at some times, but not at others.
With two masts in line, the adverse effect is when sailing to windward, just when you'd rather not have it, and on a run, when it is difficult to get the foremost sail to pull well.
With two masts side by side, you'd expect the adverse effect to be most obvious on a beam reach, but those who sail such rigs report that such an effect is not too worrisome. Going to windward, there seems to be little adverse effect if the sails are spaced apart by more than their width.