From a look at the Scope outputs, the system is indeed not stable. If you change the simulation stop time to a larger number (say 20), you will see the terminal voltage and electrical power start oscillating at around T = 13. My preliminary guess is there may be some unwanted interactions between the two synchronous machines which are causing the system to be unstable.
There are two example models shipped with Simscape Power Systems which seem to resemble your use case. You can check them out by typing the model names separately in MATLAB Command Window:
In both examples, a Simplified Synchronous Machine is used together with a Three-Phase Source (instead of another Simplified Synchronous Machine) as the swing generator. I would recommend to check out the above two examples, and try replacing the Simplified Synchronous Machine on the right-hand side with a Three-Phase Source to see if it helps improve the simulation results.