If you enable "new calling experience" in Labs on develop.element.io or Element Nightly, you should get the option to either select Jitsi or Element Call when s

If you enable "new calling experience" in Labs on develop.element.io or Element Nightly, you should get the option to either select Jitsi or Element Call when starting a video conference in an existing room. (It's still beta though, and I just tried it and it was buggy, so there may still be work to be done there.)