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.