If you leave the esc with the red pulled switched on all the time, and use on/off switch on the one supplying bec; they should both get power when battery connected, and when flip the switch to arm RX it should give the on sounds and lights on both.
I dunno about the reverse then, it's made of chinesieum? Jumper next to heatsink
What size motors are you using? Or even better, what's their stall amp draw?
3 amps, 3 ch, reverse, should do the mixing?
https://m.aliexpress.com/item/329577...Q1547788566493
Heng long tank control board
Sabertooth 2x5 amp
https://www.dimensionengineering.com...abertooth2x5rc
Sabertooth 2x12 amp
https://www.dimensionengineering.com...bertooth2x12rc