You could also just proxy a third Ghoul with a random object.
EDIT: This is completely wrong; I was conflating Ghouls and Goat Spawn. I think the correct answer is still to just have two set aside and not worry about it, but it's not such an obvious issue opened up if you were to proxy a third.
Don't do this unless you keep the two sets of set-aside ghouls separate for in-game purposes. Shub-Niggurath's 2 ghouls are important because you have two opportunities for Cultist reckonings to be replaced by Ghouls before a third will (n the absence of defeating the Ghouls) have to spawn the Dark Young. If you proxy a third Ghoul you make that easier on yourself.
I don't know why the Dreamlands board has you set aside a ghoul, but I imagine it's fine to just use it from the normal pair as necessary.