Well, I found a way to do simplify the first step.
Just click in the grey are just above the tracks, to select all tracks. Then the scissors action will affect (cut) video and audio.
But the actual trimming still seems to affect only one track at a time. Is there a better way to do this?
By grouping the separate video and audio track I get the same effect as selecting all tracks, that is I only need to use the scissors once, but with the tracks grouped, the "bracket" icon doesn't show up when I hover the mouse pointer over the scene "limits". Trimming seems disabled with the tracks grouped.