Board Thread:Questions and Answers/@comment-39463658-20190515010536/@comment-25933225-20190515200737

To inherit a Titan shifter's power, you need to eat him/her (being transformed in pure Titan is just because it's far more easier to eat someone while being in this form, for obvious reasons, and you're not conscious of it). But he has to die in the process, otherwise the power isn't transfered. The "center" of the power is located in the spine, which explains why it's the part that you need to eat, but you need to kill the shifter.

I don't think that injecting some shifter's fluid to a pure Titan will do something, if the shifter is still alive.

If the theories you're making were true, don't you think that for 2000 years of Eldian history, the number of shifters would have significantly increased? They probably have done thousands of experiences (the Ackermans), if they had found a way to double the number of shifters, they probably would have done it.