Two openHAB instances under one my.OpenHAB account?

Is this possible?
Can they share the same UUID and Secret?

No. There is no way for the service to know which OH instance you intend to communicate with. For example, if you go to, which sitemap from which OH instance should be returned?

I have one instance of OpenHab connected to my.OpenHab account and my second instance is connected to the first using MQTT. You will find discussions of MQTT connections in this forum.