A 23 spline transfercase, could a 21 spline be put behind it to double up? I understand the 23 spline input for the tranny. But what is the backside of it, the same as a 21 spline? So could you use a 21 spline doubler behind the 23 spline case or is the input shaft also have more splines where in ingages the gears in the case? I looked around here and Pirate but found nothing really, or my search skills are weak. 
the backside is whichever you want when you buy the adapter. The coupler mimics the trans output on the backside.
