If I understand this correctly, you are wanting to run a 4 channel amp that is driving 4 midrange speakers and want to know if you should go with a 2 channel input versus a 4 channel input from the bitone to the amp?
Personally, unless you are going to dedicate one midrange as mid frequencies and the other to the midbass frequncies, I wouldn't run another set of midbass drivers. The more speakers you have, the harder it is to get imaging and staging correct. That being said, I'm far more into sound quality than just being loud.
If you are adding them just to make your system louder, go with the 2 channel input. If you are going for SQ, go with the 4 channel input and assign one set as midbass and the other midrange.
Honestly though, if you are trying to get the loudest system you can, having the Bitone is kind of pointless. You could have gotten all the functions you are going to use in it for a lot cheaper and just got a decent active crossover.