Running two v3 on the same miner (AMD / NVIDIA mixed rig), starting the second node kills the first miner that's running until the watchdog restarts it. Not sure how you can get around that since when you start the miner it kills any miners that are already running.
run the another client as v2.9 until the new clients it's not ready to use.
I continue the work in the v3.0 to make it better but when @dpn is ready with the new Win/Linux client we will switch to it.