Of course they can communicate with each other, they're extensions, not web pages. The first one could act as the 'master', and proxy all the requests.
It's obvious the limiting is rate based, otherwise this would never have happened, so if it is rate based then the toolbar could pace itself to below that rate. Of course that would 'give away' the rate to observers of the toolbar during a browser restart but they could observe that just the same by checking when they get blocked, so that's no loss.
The toolbar knows I'm logged in, knows that a browser has just restarted and presumably can see how many instances/tabs are open (after all that's what it provides the info on) so it has all the data at it's disposal to make the right decision. This seems like a simple oversight to me (that a user installing the toolbar on a machine with a large number of tabs open would land in this situation).