(1) Install this Mac service to use a right-click to Make Symlinks:
(2) Create a Dropbox folder called Popclip [Same concept if you use iCloud - not really any different]
(3) Find the folder ~/Application Support/Popclip and copy it and its contents to the folder in Dropbox/iCloud called Popclip.
(4) Right-click on the Popclip folder in Dropbox and choose to Make Symlink
(5) Delete the Popclip folder in ~/Application Support/Popclip and move the new Symlink to its place. Rename that Symlink from Popclip.symlink to Popclip.
(6) On your second Mac, repeat steps 4 and 5
(7) Repeat steps 2-6 for the file (not folder) called ~/Library/Preferences/com.pilotmoon.popclip.plist