Sometimes I open a second project, or another version of the same project, in order to copy a clip or track across.  But I've never found a handy way of toggling between the two projects.  Is there a shortcut key; I can't seem to find one.
 

it used to be ctrl-tab (standard windows thing for tabbing trough child windows, but maybe skylight threw that out?

ctrl-tab work good here

Go to File and under your mouse click you have the choice to open the project's version  you like.

Ctrl-Tab only works one way for me, i.e. it won't toggle between the two, just switches to one and then Ctrl-Tab does nothing.

forget..

Ctrl+Tab should toggle - working here. An alternative is to use Alt+W to open the Window menu, and then hit the number of the window/project you want.

Skyline_UK
Ctrl-Tab only works one way for me, i.e. it won't toggle between the two, just switches to one and then Ctrl-Tab does nothing.

It will tab between all open windows, so if project #2 has an effects window open, your next ctrl-tab will focus on the effects window, which makes it look like nothing's happened. That's got me a few times too.

For me, the shortcut that toggles between the opened windows is ALT+tab.

ALT+tab will switch between separate application windows - or separate instances of applications running.
 
CTRL+tab will switch between windows in the same applications (typically this applies to an MDI - multi document interface).  e.g., if you open two projects while the application is already running, then the same 'host' application is servicing them both.  You would use ctrl+tab to flip between them.  This same this thing happens on a 'tabbed' browser.  The ctrl+tab will move between each tab.  (this is also and MDI interface - just displayed as tabs).
 
I think you can control this in CbB by selecting the open each project in a new window (or something like that).  If the application doesn't 'force' a new instance of the application (alt+tab), then it is running in the same memory space (ctrl+tab) to switch what has 'focus'.
 
Good luck!