Several points, some of which have been made above -
(a) this discussion is not related to certification, rather to a practical operational consideration
(b) same thing happens 2-, 3-, and 4-engine aircraft. Keep in mind that this crosswind effect on Vmcg can be significant. DC9 comes to mind at an increase of around 0.5*crosswind value and, I think, someone referred to a 4-engined aircraft having an effect in excess of the crosswind value - sobering thought, is it not ?
(c) no necessary relationship between Vmcg and Vmca (or Vmc, depending on your preferences). Mad (Flt) Scientist wrote up a good brief on the subject in
a similar thread which is worth a read.