Possession and Masquerade - are these too powerful in 2 player games?
Possession and Masquerade are just known to be wacky when used together.  That's just what happens when you have 100+ cards at this point.  Donald X. Vaccarino said "Possession is a case where Masquerade ends up being a weird exception", but it's basically not worth special rules or changing the card for this one interaction.

keithjgrant is also right that if Possession is out, you'd better not buy Masquerade. :)