I have never come across a low signal warning but in my mind if there was you would also require a rx that sends info back to the tx just as you would with telemetry sensors.
Now the trimming to adjust the DT doesn't sound good. Any of my twins I calibrate throttle for both esc's one at a time , totally disconnect the one you are not calibrating. Then I disconnect one of the power wires from one of the esc's and last is a simple vertical thrust test, it should lift straight up , if it doesn't there is a problem whether it is a motor, esc or prop but it should lift straight. I also program a switch so I can turn off the DT if I need to or atleast until I get the plane all trimmed in.
Possibly you esc's bec couldn't supply enough voltage so the external one is a good idea. As Psyborg said if you have both power leads hooked up that could have caused it also.
It sucks though, it looked really good.