One of my biggest Aperture frustrations is the inability to open multiple Projects or Album windows and drag and drop files between them. You can drag images from a Project or Album to another, but finding targets amidst hundreds (thousands) of albums and targets is very hard.
The “Favorites” and “Recent Items” filters in Library search can help:
Right click on an Album/Project to make it a Favorite. Anything you touch/view is “Recent”. Between Favorite and Recent filters it’s much easier to navigate between Projects or Albums in a large image repository.