Is it possible to test steam online subsystem multiplayer from the editor? (standalone)

I am working with a team mate and we use perforce.
We both sync to latest then launch standalone.
We get the steam overlay and can see eachother playing “space war”.
However when one of us creates a session (blueprint create session) and the other tries to join any sessions (blueprint find sessions → join session) it always fails to find any sessions.
I’ve got print outs that tell me “session created” etc when the blueprints succeed.
Is it just not possible? Do I just need to package the game each time?

No.

You have to fully package the game, then use a different PC/Laptop to act as server. Once it has the game server loaded and running both of you should be able to see it and connect.