It's because they are so alike that they aren't compatible at all. Both of them are attention whores who want to do things Cool and Big.

If they are both trying to run the whole show and...well, show off, then they're competing rather than harmonizing. All ego, and like Black Star keeps repeating, you can't have two stars.

Tsubaki works because she's more than happy to let Black Star lead. She has infinite patience and tolerance, at least so far. Maka and Soul work for a different reason. Maka wants to succeed, and Soul wants to do everything "cool." They are towards the same goal, but compliment each other since Maka doesn't mind being a little to the side as long as everything goes smoothly and Soul is happy to let her take the lead as long as it all looks very cool (and with Maka's skills at scythe wielding, it usually does). Maka and Soul share the stage, equally.