My only experience with trying to have things happen simultaneously, is aging up twins. The game won't allow it, as the aging Sim has to do that spin around and then depending on the age you have to select additional traits. With a wedding you don't have that, per se. Give it a try, what do you have to lose? You could always do a Save As, first, in case things go terribly wonky. Do let us know if you're successful at the nuptials. That would be cool if other players have sibling Sims who want a Double Wedding. :) Good luck, sounds like fun!