Probably redundant, but here's how I did my single battery setup, which would be the exact same thing as a double but with one less solinoid.
Key on power > solinoid.
Battery+ > solinoid.
Battery- > solinoid.
Solinoid + > all switches
Switches + > accessory
Accessory - > solinoid
If you want to do double battery, all you do is throw another solinoid and run the key on to that as well. Wire your second battery to your accessory solinoid.
I don't think there's a pigtail. I spliced and I've had no problems at all. Just make sure to use a relay or solinoid. That wire is stupid thin.
Also, adding a master switch is very easy.... Any switch works. All you do is
Bat+ > switch
Switch > output post of solinoid
Just make sure you select a switch that has no drain when in the off position. 5 pin won't work. I would get one of those jet fighter switches in green. Cool switch cover, you can't accidently turn it off, and when you want to turn it off you just close the cover and it pushes the switch off.
Sent from my Pixel XL using Tapatalk