Sounds interesting...
I've seen two in the same tank in a LFS.
I've seen videos and forum threads showing two Quoy's in the same tank.
I've heard from someone I respect a great deal that in his experience, two in a tank _didn't_ work on two separate occasions.
Mine ... four full weeks after purchase ... is still spending almost all of his time hiding in PVC pipe fittings. Got meds from Dr. Foster & Smith last night, so - woohoo! - could do a 28 or so gallon water change on the 75 gallon QT - and sucked an awful lot of goo (that used to be NLS pellets) off the bottom of the tank. I'm beginning to become frustrated.
Went to a (different) LFS to check out a Quoy's he'd posted on Facebook - he'd had it labelled as a Princess parrot, but it was sold. (He did offer a tip or two on getting my guy to eat - tried one, but it didn't pan out.) Swung past a nearby Petco on the way home ... and they had a Princess parrotfish. ("Aaah," said the Petco person, "There might be ich in that system..." "It's Petco", I replied. "Of _course_ there's ich in the system! I'm already treating with CopperSafe, so . . . ") Smaller than my Quoy's, and transitional. Got to wondering . . . would it have any effect on the Quoy's to have another parrotfish in proximity?
So now I, too, have two. The princess parrot swims about the tank, and I've seen her pluck bits of frozen food out of the water column. The Quoy's . . . mostly hides in PVC pipe fittings.
I'm still frustrated with this ... fish.
~Bruce