You can "easily" drag a panel to another monitor, size it by dragging sides/corners to just a tab bit less than the monitor's screen edges, add additional panels into it, and create a workspace.
"Easily" as in a ton of mouse clicks and futzing. Taking several minutes, for EACH AND EVERY CUSTOM WORKSPACE. (Yea, I'm a bit ticked at this issue.)
Then, of course, you have to be careful because if you go ONE PIXEL over onto another screen, Premiere will get cranky and do many odd things. We sort out those odd things back to this issue every once in a while on the main u2u forums. Several a month. Year after year.
And if, Heaven forfend, you change your monitor layout diagram in Windows, now every freaking custom workspace you've got is WRONG as Premiere clearly takes the location from the main UI window in a relative sense, it does NOT recognize the other monitors as a separate entity.
So you get to rebuild every one of your custom workspaces.
Other apps ... and yes, the R word comes into play here ... can be set to simply use monitors as separate screens. Biff bam bowey, one click you're done. Now granted, I really dislike how rigid their UI is in general.
But clearly, an NLE CAN have the ability to recognize multiple monitors as separate entities and treat them as such.
It's about time for PrPro to get that ability. Or rather, about six years AFTER PrPro should have received that ability!