I think a multi cross would last longer than a multi main because the poly main would rub along the multi. A poly cross would rub across the multi main. This would be magnified by a shaped poly cross. Somehow, this is not the case with a nat gut main. Something about the gut's oils acting as lubrication.
Dunlop MuscleWeave 200G 95, SEARCHING FOR A NEW MAIN STRING / Hexy Fiber 16 @48/53, dropweight stringer