Just Swap
If you are truely just talking about a swap -- you sit in back spouse sits up front - just do it.
Once you get past the GA scanning at the gate swap boarding passes quietly. I don't see how anyone would know the difference. If anyone were to say something just explain that you swapped. I can't see how anyone would have a problem.
Now if you are suggesting that both of you would sit in first -- that's awful tough and unlikely. Maybe you could sweet talk the FA or GA, but don't expect it.