Positions are only part of it, there's also orders, formation and opponent but putting them aside.

I've had AMx/Mx that can perform in both roles, in either 1 or neither, same with AMx/AMC, same with DL/DC that can do both, etc. I currently have 1 DC/DMC that excels in both and another that is better as DMC and average as DC (but can also do a job as MC).

Additionally, without the roles, I've had good MCs that make good DMCs and others that make good AMCs and on my OMA team I had a good striker that made a good AML and AMR. (I tried another ST as AML today and he was utter crap.)

I've even had a left footed AMR/AMC in the past that was good as AMR but crap as AMC....so I trained him as AML as an experiment and he was crap there too.

There are no hard and fast rules. Additional roles don't mean a player can play that role, it just means he can try without penalty. After all, some single role players can't play there sole role either.

If you want to add an additional role then try the player there first - even with a red penalty there should be something encouraging there, if not don't do it.