I control (and I'm also the own of) a Graf Rats which has become a copy of Gisela, the Broken Blade, and a Midnight Scavengers which is a copy of Bruna, the Fading Light. At the beginning of my end step, Gisela's triggered ability will trigger, and…
My question is, would they successfully come back and meld into Chittering Host?

My take on this is they can.
According to 701.35b, Meld action seems only care if both recipes are in a pair. The only concern is that the wording of the ability on Gisela explicitly states they would meld into Brisela, but I think that name is just there to represent the object of the melding result, just like card names on a card to only represent itself, right?

Anyway, I am hoping for a comfirmation on this or correction if I'm wrong. Thank you!

Hi,

This question was addressed a few months ago: http://apps.magicjudges.org/forum/topic/28679/

In short: the cards are exiled, and they'll only return melded if the two cards in exile (who are back to their normal forms) would make a melded pair. In your case, the two cards in exile make a melded pair, so they'll return melded as Chittering Host.

Nathan Long
Magic Rules NetRep