Hello SongOfDeath,
Anti-Aliasing (AA) refers to the smoothing of aliased, or jagged, lines when displayed on a computer monitor. This is a correction derived from what the camera can see on the post processing pass. It is not possible to filter out and assign half of a screen to render FX and the other Temporal. There are ways to use the ‘use custom depth mask’ feature from a HighResScreenshot. This will give you an alpha but is a purely binary image with no aliased greyscale values. So the RGB values are aliased but not the alpha. This way is fairly useless for compositing/postprocessing. This is also a fairly inefficient method as the masking happens after the Anti-Aliasing is done. This may be useful for a single image/screenshot but not for gameplay.