From document I know that a Pod must be scheduled by one scheduler. Such as scheduler A schedule pod a and scheduler B schedule pod b.
The question is that: can a preempt b if b is already running? Which scheduler will do this operation?
As far as I am aware Pods are not evicted by schedulers at all. It's kubelets job to nuke pod if it ie. no longer fits in the node or something similar. You do have a Descheduler if you want that, but that is a completely different piece of the puzzle. Your own custom scheduler could also implement descheduler logic of some kind if you want to merge this, but then you'd need to make sure on your own that descheduling logic does not collide within your multiple deschedulers.
To stress again, this is not a part of the cluster builds by default.