I know. Two new features in the space of a week. Don't get too used to it.
P2 now has a 'Show floating window' checkbox in the Background menu. Select this, and you'll get a second set of imagery in a floating window:
The window follows your main cursor location. So you can have Bing in the main window and Ordnance Survey StreetView in the floating window, or a blank background in the main window and Bing in the floating window, or whatever you like.
There's a "lock zoom" checkbox to stop the floating window zooming in or out - for example, edit at z18 with Bing but see OSSV at z16; or edit at z14 with Bing but have the floating window at z19 for a close-up view. It also respects the "max_zoom" parameter of the editor imagery index.
I coded most of this at a hack weekend a year ago but had never got round to finishing it off. Hope it's useful.
At the same time, P2 now makes a passable fist of rendering multipolygons where the tags are on the relation rather than the outer way (yeuch). It's not perfect and it won't do any of that crazy "advanced multipolygon" stuff. But it'll do until we finally get an area datatype.