You are able to raise three corpses under the same requirements as Dead Duet and you may fit three corpses meeting the requirements into one hexed jar.
When you Raise the Dead, you can raise two corpses instead of one. These corpses have to be one size category smaller than the corpses you normally are able to raise and deal each one die category less damage (d4 instead of d6). The Power you create with Raise the Dead is shared between both corpses and you may fit two corpses meeting the requirement into one hexed jar.
You craft a familiar out of an intact part of a corpse. This could be a Floating Eye, a Crawling Claw, a Speaking Skull or something similar. This familiar will provide you +1 ongoing to a basic move (e.g. Discern Realities the Eye, Hack and Slash the Claw, Spout Lore the Skull) while it’s with you.