Is it possible to have two instances of ES searching the same search peer pool?
It's 'possible' to install two, but I would not recommend doing so. The impact of running two ES Search Heads can have a catastrophic effect on the peers as you're running double the data model accelerations, plus all the other scheduled searches ES runs in the background.
If you want to set up a second ES node for testing (or similar) purposes, it's best to place it on a separate environment.