Yes that is true what you said. As I said both NodeMCU´s power up just fine when I use those "VIN"-pins to feed the 5V power. And yes you can connect USB-cable into it "on the fly" for programming.
Actually, if I first start up the NodeMCU´s via "VIN"-pins and after that insert that extra 5V USB-cable to the one NodeMCU (on the fly) everything works fine. Relays start up.
So why those Relays does not start up, if I use only those "VIN"-pins for feeding power? Why it needs that USB-cable for relays? What I´m doing wrong I just dont get it. Ground is now connected as you said.