As I test, I do not see a way to mask only one AV track, but I do not think BEYOND can output to two seperate video projectors (unless they are set up as EmuLasers and then they would not output video from an AV timeline track) so I cannot picture a way masking only one AV track would be benificial (at this time).
Can you elaborate on what you are trying to do? Maybe this is something that needs to be implimented or, maybe this is something that, even if we could do it, is not possible given BEYOND's video output capabilities.