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.

Great info. I am having problems with my Smitt Rotterdam. This looks like why I am having trouble. I have both red wires connected one from each ESC.

Thanks everyone for your input, your replies gave me confidence to remove the red wire, which I have done and completed the rest of the wiring. All works perfect, 2 outer motors on left stick forward and reverse, centre motor on right stick forward and reverse. The servo works centre rudder which in turn is coupled to the port and starboard rudders, off the right stick side to side. All worked perfectly until I noticed the outer props were turning the same way,easily rectified I just swapped the wires from one side of one motor to the other, this was because they are left and right hand props. They turn over really smooth and when used with the centre motor should give a good turn of speed. The centre motor on its own should be sufficient for docking and slow manoeuvres.now all the wiring is done I have fitted the deck after first drilling and plugging 4 holes to facilitate the removal of the 4 outer motor mounting screws as these are inaccessible once the deck is sealed and fitted

Hi
If your esc has bec, then you need to remove the red wire, so with two esc you need to remove one red wire this is so that your RX does not get two power feeds, if you leave it as is then you could just burn out your RX, (don't ask how I know)
Hope that this help's

Fred

hi.
Assuming you are powering the rc reciever from the esc you have to disconnect the positive from one esc.
so there is only one source of power.
i am not sure why you have to do this but someone will know.
Have you tried google.

As per SeamanCook's response just one red wire to the RX.

You need just one red wire.