Local testing via Unity's Multiplayer Play Mode (MPPM)

MPPM is Unity's official solution for local multiplayer testing, available for Unity 2023.2+.

Pros

Short iteration times
Tighter integration within the Editor, doesn't require multiple (standalone) Editors open

Cons

Requires Unity 2023.2+
Can be more resource demanding than other solutions - this might change in upcoming MPPM updates

How to use?

Install MPPM as described in their Installation Instructions
Open Window > Multiplayer Play Mode
Enable up to 4 Virtual Players
Make sure that the bake data is up-to-date before starting to test, and that the Replication Server is running with the latest schema generated by it
Enter Play Mode

coherence tells apart Virtual Players from the main Editor, so it's easier for you to not edit assets in clones by mistake.