I think the idea behind only one BEC connected to the reciever is linked to having one battery with two ESCs connected to them.
ESCs are never perfectly the same, so one will have a BEC that is supplying slightly more voltage lets say BEC2 is 5v and BEC1is 5.1v. The receiver doesn't care, it just needs a voltage above its cutoff, so it takes power from BEC1(cause its higher voltage pushes harder). That BEC2 is hooked up to the same pin, so instead of supplying its 5v, its now at a -.1 net voltage, so begins getting backfed to the battery by .1v from the other BEC1. Eventually, because the circuitry inst design to be backfed, BEC2 shorts out, creating a massive load for BEC1, which then also fails, cutting off any voltage supplied to the receiver.
there are ESCs that are designs to detect this and disable their BEC, Flight controllers that can choose between two different power supplies, and receivers that have isolated power, but for most of the hobby grade stuff, there isn't any protection for this. Thats why you always hear about BECs "fighting each other".
Doesnt matter which channel their plugged into, as most receivers are designed to be powered from any channel, so BECs can override each other from different channels.
