I know this sounds like voodoo but if you want to split them do this: Amnesia Prime and kill Vyemm. Do not loot Vyemm's box. Kill Prime. Get loot from both. This will work every time, you can rezz your coercer lol. I'm assuming you aren't trying to kill Prime first, that's just a road to headaches, much better to do Vyemm first or both simo. The last 5-10 times we killed Vyemm we did exactly as described above and it worked every time, zero healing from Prime.