This would be a handy feature to have. I want to be able to test a networked game with many players but only have 2 computers readily available to test with.
You can run one server and multiple clients pretty easily in one game, you only have to set up your input correctly so that each client world receives its correct input.
If you just want to test a normal multiplayer game then you can also build a client and start the client as many times as you want.
The issue with running multiple clients in the editor is that only one client world can be (conveniently) presented on screen. All other client worlds will have input lag because the prediction of those worlds isn’t presented. If the server isn’t also running on the same computer, the input lag is painfully noticeable.