When the resolution drops to 900P is the total resolution, or on both perspectives (900 P + 900P)?
900p+900p is 1800p. It doesn't work like this.
You have a 16:9 frame of some resolution.
By default it's 3840x2160 on XSX (what is called "4K" these days).
This frame can then be split by some percentage between two different cameras rendering two different viewports*.
It means that you still have a 2160p frame which is rendering 1920x2160 for one viewport and 1920x2160 for the other.
When the res go down it goes down across the whole frame.
So if the res is being dropped down to "900p" (which is 1600x900 actual resolution) you're getting two viewports rendered at 800x900 each.
(* Or "worlds". I'm still puzzled by why its being brought up since the game's unveil as being different to what, for example, Hitman games were doing back on PS2/Xbox/PS360 when they've had a similar split view. Fahrenheit on PS2 is another such example. Same assets reuse, sure, but this is RAM requirements, not rendering. So fundamentally this game isn't doing any wonders when rendering two "worlds" and it certainly can be done on any previous gen h/w. )