BEC receivers are supposed to withstand a higher voltage than normal, but presumably the internal BEC supplies a safe voltage to the internals of the receiver. Whether this voltage is supplied to the items plugged into it is uncertain. If it is, supplying a higher voltage to the controlled output pins as shown might not be good, unless the BEC has been specially arranged, it will not like having a higher voltage applied to its output, if not then OK, the regulated voltage just gets processed down to the RX circuit.
But, as per the drawing, if 2 batteries are connected in parallel, precautions need to be taken. They do their best to even out their voltages by passing as much current as they can through the connecting wiring. In theory, this might be too much for the battery. In practice, the weakest bit will be the printed wiring in the RX connecting the pins and/or the connections to the pins, which will heat until either the batteries even out, or the weak bit melts, whichever comes first.
The big trouble with lead acid, apart from the poor weight to energy stored ratio, is that they can deliver much more current than they can handle without damaging themselves. The 3AH/20H quoted are capable of 150mA for 20 hours. A sailwinch working will go well over that, so I would go for NiMH which is well capable of doing the job without damaging itself. If each one is locally powered, then multi-ESC rules apply - disconnect red wires as appropriate.