Lipo battery auto low voltage switch.

Hello all,

This is my first post here. I'm a new arduino tinkerer, I'm full of ideas but lack the proper programming skills and frankly I'm circuit-illiterate. I'm probably going to be told to shove off, because my hand will probably need to be held.
Anyways, currently I've built a full carbon fiber fpv 290mm hexacopter running arDUPILOT APM2.8 with ublox 7m GPS and 3DR 915mhz telemetry radio and a fpv 5.8ghz system with mavlink and minimosd as well as a good deal of LEDs.
I'm not sure if that matters at all, but I figured I'd leave no details unturned.
Anyways, currently, I have 6 turnigy nanotechnology 11.1v 25c 2000mah lipos they are long and slim and fit very nicely.
What I'd like help with is using one of my pro mini 328 16/5 and some of my i2c voltage sensors to create a device that has 2 or possibly more if weight allows, we will call them B1 and B2.
Anyways I'd like to possibly use the voltage sensor to monitor B1 voltage during flight and then when a user defined low voltage is met , typically when a loss of flight power is noticed, it would switch to B2 and then perhaps trigger a buzzer at B2 LV.
Another idea would require a more reactive approach watching the osd voltage and using a switch on one pro mini with a 2.4ghz transciever at the ground control station to tell another 2.4ghz transciever/pro mini onboard the drone to switch from B1 to B2.
Also if this product already exists, depending on price, I may just buy one, but I have 7 pro minis and a few voltage/current sensors.

Anyways. If anyone could offer some serious layman's help I'd really appreciate it.
Anyone who would be willing to engineer and sketch the device and present it to me in an idiot proof format, I would be able to compensate/donate through PayPal or online orders.

Please help

It would seem to be much simpler to wire the two batteries in parallel and have a somewhat higher voltage detection threshold - sufficient to give a safe return to base. The overall energy economics will be identical.

You could NOT parallel the low battery with the charged battery because there would be a huge current flow. And I presume you could not disconnect B1 before connecting B2.

Switches also introduce unreliability.

With a simple voltage divider an Arduino could detect the battery voltage with one of its analog pins.