aircrash

The reason is that a BEChas a voltage regulation.
If it did not had any then it would not matter if you connected two or more in parallel.

As there is a regulation circuit (actually a chip, which chip is a circuit by itself) and the fact there is no chip the same, so regulation differs, then one BEC sees a load and regulates the voltage and the other BEC sees an expected desired volatge and does not do anything, something like it shuts down its rest system (power supply).
Now if the first BEC runs out of power due to a high load which means high current demand, more than the BEC can supply then the other BECstill will not power but only after some time. So there is a voltage (and power) drop anyway.
The biggest problem is the above can happen all the time. As it has to regulate the voltage to a point when there is a slight difference then the second BEC tries to take over. This confuses the first one and goes to sleep and vice versa and all this time this happens there is power hickups.

I hope you got the idea.

So... the only thing you can do to make your power supply better is to have a suitable BEC.
One that can handle (output) a little more current than the actual max needs, so it never reaches the limit and it powers all the time no matter what.