How to enter play mode on two separate Unity instances at the same time?

Hi,

I'm trying to implement online multiplayer in my project with MLAPI, and I've cloned my project with https://github.com/hwaet/UnityProjectCloner and it works well, so now I have two Unity instances open, one with the GameProject and one with GameProject_clone. If I press Play on one, it will enter play mode. However, when I press Play on the second Unity instance, it will stop play mode on the first instance. Wondering how to let it run both at the same time so I can test multiplayer. I can see the guy in this video does it so I'm wondering why mine quits. https://youtu.be/ZiKW5vXPGz0?t=365

Is it normal behavior or is it because my project uses OpenVR or something? I'm using Unity 2019.3.2f1

Cheers

Tl;dr: pressing play on one instance stops the other. How to not stop?