how what works? the 2 engines render side by side according to the developer
That's not how it works...
H2A is running higher res textures, geo, characters and animation AND running the OG engine at the same time
He then revealed exactly what Halo 2: Anniversary is running simultaneously, before suggesting that if the game did not run the original engine the resolution could, in theory, be boosted:
"Two game (graphics) engines - the OG H2 and H2A, and the original audio (music and FX) and completely new music and FX. And the switch is instantaneous. If it weren't running the OG engine it could in theory run at a higher resolution but that's not the intended nature of the project. It's designed to be a remake that lets you switch between the two instantaneously. Now you can feel one way or another about that, but that is indeed the intent."
and actually, halo 2 classic does run in 1080p, so the game is rendering both, a 1080p frame and a 1328x1080 frame 60 times/second (3248x1080/60).