Folk don't seem to understand this problem.
You would never connect a 2 cell battery directly in parallel with a 3 cell battery. At best one would charge the other and at worst there would be a fire.
You can regard any one ESC like a battery. So joining the two output red wires is the same as joining two batteries.
You would not expect any problem because the two ESCs nominally give the same voltage.
But the electronics inside can try too hard to have their own way.
For example one ESC might be giving 4.99V and the other 5.01V.
On their own either is legal and OK.
But put them together and one tries to push the voltage up and the other tries to pull it down.
Neither is happy because neither can win.
Worse, if there is some clever software in there or any circuitry that senses an error, the only safe thing for the ESC to do is to shut down and maybe try again. There is a lot of potential to damage the included H bridge (google that term} and if the power supplies are not right the risk of severe damage is too high.