if I were to buy/register an 11.5 inch barrel and a suppressor?

Yep, sure do.  Two different forms, two different $200 tax stamps.  

Unless the supperssor is internal to the upper, but if it detaches, yes.

Quoted: Yep, sure do.  Two different forms, two different $200 tax stamps.   Unless the supperssor is internal to the upper, but if it detaches, yes.

Actually thats not quite correct; if the integrally supressed upper were still short of the 16" limit, you'd also have to have the lower registered as a SBR.  In the days leading up to the May 19 cut-off, BATF did allow a few MP5SDs to pass as one-tax guns, but later changed their stance and required two seperate registrations for SDs (one for the MG/SBR, one for the supressor).

Don't we pay ALL taxes twice (one way or another)