First you have to tell LR to treat JPG files as a separate file. You do that in the Preferences under Import Options.
Not sure if there is a way after they are imported without that option checked to separate those JPGs from their DNG file. You can try checking that option and see if the JPGs then become separate files (Actually they are separate files and LR is just showing them as combined with their accompanying DNG).
But I must ask why do you want to edit the JPG instead of the DNG?
Thanks, James - that was the solution! After changing that preference, i can see the JPG as an independent file.
Use Synchronize on the folder that has the raw+jpgs that are stuck together after you enabled the treat-separate preference and the hidden jpgs and any other unimported images will be offered for import.
I usually export my jpgs (or TIFs) to a subfolder of the master photos, so they aren’t mixed in unless I enabled show photos from subfolders.