AFAIK, Ymir was a mindless Titan who became a Titan shifter by eating the Titan shifter Marcel. What does a Titan needs to eat from another Titan shifter to steal its power? Does it need to eat it completely, its human part, part of eat, it works by only biting, how does it work?
Titan ability resides within human's spinal fluid, so it is enough to consume ones spinal fluid to get titan's power (chapter 65 of manga).
It is not clear what would happen, if person will consume only part of spinal fluid of shifter titan.