Back in 2012, Jellokittie had this exact same problem, asked for step-by-step instructions, and never got them. While confusing, the reply (and the manual) do however give some clues , and I somehow managed to figure it out. So here’s the answer, only 5 years late:
Let’s say you have a project with 4 tracks, and you need to time-shift only the bottom two. Click Tracks > Add New > Label Track.
A label track will appear at the bottom. At the far left side of the screen, click on Label Track > Move Track Up. Do this twice (or grab and drag with the mouse). Click somewhere inside the label track, then click Tracks > Add Label to Selection.
This isolates the top 2 tracks in their own group. Now, do it again: Click Tracks > Add New > Label Track.
A label track will appear at the bottom. Click somewhere inside the label track, then click Tracks > Add Label to Selection.
This puts the bottom 2 tracks in their own group.
Now click Tracks > Sync-Lock Tracks (on/off) so Sync-Lock is on (a check mark appears).
To get the bottom two tracks to move together, now you SELECT the bottom 2 tracks with the mouse (this is why only adjacent tracks can be Sync-Locked). Little stopwatch icons should appear in the bottom Label Track.
Now, click on the time-shift tool (top of the page, center left, just under the hourglass), grab track 3 with the mouse, move it, and track 4 will move also.