The issue of internal BEC's is a very important one as most these days are switchmode, and if you leave them connected in parallel with each other one will kill the other, switching circuits do not like to be run in parallel. As mentioned remove the red wires from each ESC lead, NOT from the Y lead.
Connect each ESC individually to your Rx and do the throttle range setting, IE throttle full, Tx on, plug in battery, ESC will beep, close throttle, ESC should count cells then arm, throttle range end points are now set to your Tx. Do same to other one then reconnect to Y lead, should be close to each other unless something is wrong with motor Kv or one ESC has an issue.