My apologies, I moved it since the OP didn't ask about agency, I assumed it was an incorrectly placed thread.
As a RAID instructor and currently busy with OC Deco50 (the first doubles course) I cannot find anywhere in the standards that stipulates the manifold must be behind the head, if you can find an instructor happy to teach that way then as long as you can meet the required skills there should be no problem.
Is there a particular reason you aren't looking at sidemount? It seems to be the go-to for those with mobility issues with standard BM doubles.