Thanks for that Phill. I still don't really understand. If both BEC supply say 5 volts through a y lead isn't that still 5 volts?
You can see by my questions that I don't really understand electrics, I usually just follow instructions and things seem to work.
Each BEC is generating a nominal 5V but there will be a tiny difference in their '5V' voltages due to internal component tolerances. Each regulator circuit looks at its own '5V' output and adjusts itself to keep it constant regardless of load current up to its maximum output.
If you connect 2 together, each will fight the other trying to correct their individual outputs to what they think is 5V - they will push more and more current around the loop until they overload each other - things will get HOT! If you're lucky, one will shut down first because of good circuit design - if you're unlucky, one will blow up and the other may be damaged.
If you're really unlucky, both will be fried.
This will happen in a couple of seconds - you won't be fast enough to stop it and the smoke and heat you see and feel on the outer case of the ESC is nothing compared to the destruction inside the unit.
Cut the wire - it's much cheaper!