When working a multiplayer game you need to be able to quickly test and iterate over your game, there are few ways to do this.
Editor & Build
The easiest way to test your multiplayer game is to make a standalone build (PC or Mac) then run the editor and the build at the same time.
It's recommeded to make a development build that way you can catch errors easier.
Editor & Clone
This is my favorite way for testing a multiplayer project. You basically have your original project then create a clone out of it the best way to do this is by using by using ParrelSync.
ParrelSync is a Unity editor extension that allows users to test multiplayer gameplay without building the project by having another Unity editor window opened and mirror the changes from the original project.
Test project changes on clients and server within seconds - both in editor
This is the recommended way to test your multiplayer game, this allows you to debug and catch errors way easier than any other way.
If you are on Mac or Linux you should get the repository from the Develop branch.
Support for these platforms is on beta.