You can, actually. With the first piano roll (or score) window open, make sure the link icon (yellow chain) in the top left corner is NOT checked. Now go to the Window menu and open a new Arrange window. You can select a different track and open its piano roll or score window in that Arrange window and the first one will still display the first track you were editing.
It might take some clicking and dragging to place these windows on your screen real estate for optimum editing, but once you've got them how you like them you might want to assign a screenset to the arrangement you have, by selecting Duplicate Screenset from the Screenset menu and assigning a number to it.