I am making my first stacked bc cake which the customer is picking up at 10 am tomorrow for a 3:00 party. My question is.....how long in advance can I stack them without having any issues with the cake? It is a fairly simple design...12" and 9". I am working on it now...is it ok just to go ahead and stack now or should I wait until tomorrow morning before it is picked up?

Wow! working late huh?? (or early rather) me too
I don't know this as fact, however I have stacked cakes in advance and haven't had a problem. Mine were more like 9" and 6" though. I think that the main thing you need to think about when deciding to stack is storing (if it needs to be in the fridge is there enough room for a stacked cake?) and transporting the cake. Like I said, I have stacked my 2 tier cakes in advance many times before. I can't really see why, but if this is a "no-no" then someone please correct me! thanks! and I hope I helped

I would wait until tomorrow morning before stacking them. There's less chance of things going wrong

If the customer is transporting a stacked cake, I would also make sure to use a center dowel thru both cakes.

Thankyou so much!!!