The two systems are interconnected for achieving economy, reliability and security. If they are connected by a synchronous link (same voltage, phase sequence and frequency– AC link) then the frequency of the entire system has to be same under steady state. They can also be interconnected by an asynchronous link (HVDC back to back connection) where the frequencies could be different.
When power sources are not synchronized , there are instances where there is a voltage difference at the same very node where the three sources are connected .Now Mr Kirchoff told us that such a situation is theoretically impossible. What happens in reality is that there are currents formed that circulate to and fro from the power sources , and given the mag of voltage in your question the power dissipated by those currents will fry the coils and you don’t want that!