set up two different journal files (which also log different things):

journalctl MESSAGE=a -f -o export | systemd-journal-remote -o logtest/a.journal -
journalctl MESSAGE=b -f -o export | systemd-journal-remote -o logtest/b.journal -


now open the directory with

journalctl --dir ~/logtest/ -f

and log some different messages to trigger writing into both file

logger a
logger b
logger a
logger b

the output journal only prints:

Mär 25 16:06:37 virtual-fedora-sbs tpiadmin[4266]: a
Mär 25 16:06:38 virtual-fedora-sbs tpiadmin[4270]: a