to add to the orginal question, ..say you got a 23 spline AA adapter and a MC 23 splinecoupler...could you run 2 transfercases with 23 spline or does one have to be 23 spline and one has to be 21 spline? just one has to be forward and one has to be topshift to make the duals work?

Your front transfercase input will have to match the transmission output.  L, G, and W transmissions are all 21-spline outputs.  R-series trans are 23 spline, so if you have an R-series trans then yes, you could run two 23-spline cases.

FYI, the couplers sometimes vary between manufacturers, so a MC coupler will not necessarily work in an AA adapter.  It might, but it might not.