"Cycle to previous or next item in active viewer (for example, cycle through open compositions)
Shift+, (comma) or Shift+. (period)"
You'll need multiple Comp Viewers open for this to work and to have the keyboard focus on the Comp window. It doesn't appear to work when keyboard focus is on the Timeline.
Note that if you open another Comp viewer then dock them together you may need to unlock one of them for the Timeline to switch between Comps.
Here's how it works:
SwitchComp