Why could falco become part beast titan if Zeke was still alive?

I once had a theory that both Armin and Erwin could have been saved if they used half the serum on each of them, cut Bear turtle in half and then throw part of his spine in both their mouths.

However most people said it is impossible to have 2 shifters of one of the 9 titans at once. If thats the case why did Falco inherit beast? Shouldnt he have just inherited jaw because Zeke was still alive? Also if titan powers cant be split then how did we wind up with 9 shifters in the first place?