Souleater's on the right track, but I see it kinda differently
Jake is a natural leader, but he fights for those he loves, so you'd end up with one Jake who cares but really can't do anything, and one who can do anything but doesn't care.
Tobias already is split: human and hawk. We'd probably end up with one that has his human mind and the other with his hawk mind. What would make for an interesting situation (in my opinion) would be if the hawk mind ended up with the human emotions.
Cassie's would be the most subtle split. Cassie has strong morals, and is protective of what she cares about. These two qualities combined give her the motivation to fight. So we could end up with a moralistic Cassie and a protective Cassie, neither of whom wish to fight and neither of whom would want to be put back with the other for fear of becoming the killer again.
Marco is both the humour and the logistics of the group, so this would be the most obvious split. What could make for another interesting situation is if the logical Marco ended up with all his caring. Then we would have one Marco who would always be making jokes with no regard for anyone or anything, and one who could make perfect plans but would never be able to carry them out.
Ax is a follower, but proud; this is where the split may occur. We would have one Ax who had no time for the rest of the team, purely focused on his own agenda, and one who couldn't do anything without the approval of others.
There is, of course, no real logic behind my reasoning: we're working with a concept based around a random event. However, from the perspective of these being fictional characters, and incorporating the main idea from the book (that both halves are useless, you need the whole), I think these are the best choices.