I found aemaem's post got me close (and directly editing the config files in .config/xfce4/... didn't work for me), but if you actually want to easily duplicate across monitors, there were some extra steps not listed out there:
from the Panels menu > Backup and restore > Select "Current Configuration" and "Save" and "Export", which saves off a tar.gz of your configuration. Open that and find the "config.txt" file. This will contain a list of "panels" and "plugins". You'll probably want to copy this, then copy the lines beginning with "/panels/panel-x/" that you want to duplicate, changing the number (i have two panels, panel-0 and panel-1, so i created two more, panel-2 and panel-3). you'll also need to find all the lines beginning with "/plugins/plugin-y" for all plugins listed in your "/panels/panel-x/plugin-ids" line(s), and copy and renumber those as well (it seems there are sometimes issues with multiple panels pointing to the same plugin ID). be sure to update the line that lists out all the panels as well. Then resave this file into the tar.gz, and duplicate and renumber any "launcher-n" folders in the tar.gz as well. finally, you can either re-import the tar.gz in the same place you exported it, or with "python3 /usr/share/xfpanel-switch/xfpanel-switch/xfpanel-switch.py load /path/to/yourfile.tar.bz2" (from here) See the example below.
my original file:
/panels [<0>, <1> ]
/panels/panel-0/...
...
/panels/panel-0/plugin-ids [<1>, <2>, <3> ... ]
/panels/panel-1/...
...
/panels/panel-1/plugin-ids [<10>, <11>, <12> ... ]
/plugins/plugin-1 ...
...
/plugins/plugin-2 ...
...
/plugins/plugin-21 ...
became:
/panels [<0>, <1>, <2>, <3> ]
/panels/panel-0/...
...
/panels/panel-0/plugin-ids [<1>, <2>, <3> ... ]
/panels/panel-1/...
...
/panels/panel-1/plugin-ids [<10>, <11>, <12> ... ]
/panels/panel-2/...
...
/panels/panel-2/plugin-ids [<22>, <23>, <24> ... ]
/panels/panel-3/...
...
/panels/panel-3/plugin-ids [<31>, <32>, <33> ... ]
/plugins/plugin-1 ...
...
/plugins/plugin-2 ...
...
/plugins/plugin-21 ...
...
/plugins/plugin-22 ...
...
/plugins/plugin-23 ...
...
/plugins/plugin-42 ...