I would like to be able to test multiple audiences on the same page, to see how the activities associated with those audiences would appear to a user in multiple audiences.

The Optimizely preview helper would be a great way to do this. In the override, it currently only shows the audiences associated with the campaign being previewed. If this allowed for the override to allow for any audiences to be selected, and hence allow to test for how it might look for a user who is in multiple audiences, it would be extremely helpful.