Since at least 2010 Aperture has been able to display multiple Projects side-by-side in a tabbed UI by option-click on Project name. I was amazed to learn about this a few weeks ago. Why isn't this prominently discussed in Aperture's manual or help file? Why, even after I showed it worked, is it so hard to find documentation?
I mean - this is big. I've been looking for it for years, missing iPhoto 9's easy ability to split and merge Projects and move images between them. Aperture's single project focus is my biggest complaint. At last I can move images between Projects ...
No. You can't.
Which is probably why Apple has never documented this feature -- it's obviously only half-built. Rather than pull tabbed albums projects from the release Apple left them in, but removed documentation. Sad that in some fine Aperture updates since 2010 this feature was never completed.
Maybe in Aperture 4.