Without knowing the exact working of both BECs involved, the only safe answer is "don't". No doubt there are specific instances where multiple BECs can feed into a common supply, but there are still plenty out there which react badly to being presented with a wrong voltage on their output.
Having just one BEC working the system avoids the possibility of having to buy replacement parts. The "unused" BECs are still doing work - they supply the 5 volts that the input end of their ESCs need.
[size=0px]Staying within this thread , I hope, do I need two batteries when using two brushless Esc's on twin brushless motors?[/size][/size][size=0px]If no, what would the connection be.?N[/size]
Depends on the battery or batteries, and what they are capable of supplying. If the two motors, at full load, want more current that one battery can supply, it won't work.
One big battery per system, either a suitable Y lead for the power or a proper distribution panel.
One suitable battery per ESC, just plug each ESC into its respective battery.