This is a stand and fight issue. Do NOT cave on this one.
It is the silliest idea ever. One day two jealous controlling broads were planning a shower, probably bitching about how their boyfriends/husbands were going to go drinking or watch football while they spent the day at the shower. Thus, in just another attempt to assert control, they basically created the "Jack and Jill" theme so that they have their significant other by their side the whole time.
My wife knows where I stand on this (she thinks it is foolish too) so she would never ask, but I see buddies getting roped into this sh*tshow every now and then. Stand up and say NO.
