Do you want to easily test client and server of your game in multiple scenes? Do you need to open the same project in multiple editors? It’s possible and it’s easy using a very simple trick. Read on for more information http://nooparmy.com/index.php/products/educational-materials/24-opening-a-unity-project-in-multiple-editors-for-multiplayer-development.html
It works, we use the same method (as we have a multiplayer game under development), but it doesn’t always work so great over network (Windows built in Network sharing). Don’t really know why, but sometimes it is super slow (and yes, we are talking LAN here). For those cases we use Dropbox (and if you do, remember to disable sharing of the Library folder!).
Oh, and don’t try to edit the same scene in multiple Editor instances 
And you need to leave the Unity Editor and re-enter (not turn it off, just switch to another application) for most changes to actually be loaded.
Yes if you modify same asset (scene or anything) in both editors then in serialization time they’ll fight for it.