It is possible to show two browser or filmstrip windows side by side in the following way:

In the Library Inspector select the two albums or projects with the Option/Alt key pressed. One album plus one project is also possible.

Immediately both choices are visible as split browsers/filmstrips with tabs showing the album/project name and an x for closing. The dividing line can be dragged to left or right and the magnification of each part can be adjusted individually with the sliders at the bottom.

In case you want to change one of the choices, don’t forget to select first the appropriate window by clicking in it, otherwise you will change easily the wrong one!

When you want to move an image from one album/project to the other, it is not possible to drag the thumbnail image directly to the other side, you have to drag it to the destination in the Library Inspector.

A bonus tip: it is always possible to view the content of as many albums and projects as you want in one window by selecting them in the Library Inspector with the Cmd or Shift key pressed.

Editors note: There's a post about this on the site going way back to September 2010, and it may have some more info on the topic. Check that out here.