Is there a way to make violet hold less random?

[halloween168]

I'm trying to farm a specific item from it but it has only been picked once out of 7 runs, the other 6 were the same 2 bosses picked.

Edit: or at least know who its gonna pick before killing 6 portals.

[DemiJam]

its random i think... but once i tried to reset it after killing a boss... and the 2nd boss becomes different.