[Gong] Ortlieb Back [& front] rollers Each, come with a shoulder strap.., you can, or at least I did,

used a couple short strips of webbing, to connect the 2 panniers together , and so stay paired .

It was easy to do.
the webbing went over the top rail and is held in place with the screws that hold the to rails to the bag.

Ortliebs new shoulder strap pad velcros on, .. and so both straps can act as one.

anyhow.. the back pack conversion is for One of Them. the rack hooks clip onto it.
then the other would be on the shoulder strap.

Add: this Evergreen? you might seek advise for the equivalent of a school locker, on campus.