I am playing around (no conclusions yet) on loading film in my Combi back to back to double up the processing rate.
However, I am using drying screen mesh (you could also use fly screen mesh) to keep the two sheets back surface separate.
There is an anti-halation layer on most sheet film - so the chemicals need access to it to remove it.
I don't use TMax - but if you are, you may find more difficulties with getting it fixed properly.
After playing around in the light, loading 6 off sets of double film sheets seems to be a bit risky (or rather too risky for me) - so I load the outer and inner slots on each side and leave the middle slot empty.
Two sheets of film and an intermediate spacing screen make the film considerably stiffer to load - so make sure the guide slots are nice and smooth - I can see problems with scratched film emulsion without extra care.
I use ID11 at 1+1 - so I have plenty of developer capacity but you might want to check the date sheets for what ever you are using.
Make sure you wash your films properly - there is only going to be a very flow through the mesh - I am working on the minimum of at least doubling the washing routine - I'd hate to find in a few years time that I have skimped.
I’d be interested on hearing your experiences