This set up is also extremely easy, at least as far as I've seen. If they play really aggressively and rush in you can probably get a sap off and open on one of them. If not though, you want to get out of the way, and avoid a flare. Your priest should avoid getting hit by viper sting.

You have to check the buffs of the Hunter and Priest to see which has fear ward. You want to kill the one with fear ward. Getting a sap off is a big plus, and often possible. Pretty much just Sap->Fear->Blind the Hunter, if you're killing the Priest. You can sometimes get a Vanish->Sap after the Blind, which is very GG. If you're killing the Hunter, Sap->Silence->Fear->Blind->Fear. If you have the chance, you might need to do a ninja gouge or kick to stop a heal. Due to frost traps, though, that's usually not an option.

All in all, you shouldn't have much trouble downing one of the two as Prep. Once one is dead, your priest will probably be OOM or near it, and should pop out to heal/drink.

While fighting this set up, if killing the Priest is giving you trouble, you could probably just stay on the Hunter for so long that their Priest goes OOM, and you just pretty much 1v2 them. Really not too tough of a fight.