You can configure TortoiseHg so that it opens a new Workbench instance for each repository instead of multiple tabs in one Workbench instance.
It's in the "Settings" dialog, under Workbench -> Single Workbench Window
Note that it was added in one of the newer TortoiseHg versions (don't know which one exactly) and that it's not visible on the screenshot of the "Settings" dialog in the docs I linked above (I guess the screenshot is too old).
EDIT:
Here's a screenshot from my machine: