Hello,
It sounds like you simply want to see V1 underneath while adjusting a mask on V2 (a temporary “see-through” reference).
If you’re masking in the Color page (Power Window): a Power Window doesn’t make the clip transparent. Quick workaround: go to the Edit page, select the V2 clip, Inspector, Opacity and lower it (e.g. 50%) while you position the window, then set it back to 100%.
If you’re masking in Fusion: make sure you’re viewing the composite (a Merge node with V2 as Foreground and V1 as Background) and adjust the mask while looking at the Merge output.
Best,
Ivan Eldredge G.