Welcome to Tom's Forums!
The versions are different IC's and may not be compatible. If the CAS or Speed is different than I know they're not guaranteed compatible.
CM2X2048-6400C4DHX I would assume the versions is the number of sticks. Q2X8G6400C4DHX is four sticks of CM2X2048-6400C4DHX.
TWIN2X4096-6400C4DHX is two sticks of CM2X2048-6400C4DHX.