If there are two Goblin Snipers in the staging area do they prevent each other from being optionally engaged?

Yes.

They do, unfortunatly.

So one could theoretically be unable to engage them till hitting 48 threat while taking two damage every turn. Ouch!

Better to have Dunhere for that rare case! Otherwise a Gandalf blast could do the trick.

Anodos said:

Gloin + Daughter/Self-Preservation = Cha-ching!

Dam said:

^ = a classic example of turning the enemy to your advantage