ok, you need to solder it to the esc. Red to positive and black to negative. Since the batteries are connected, their voltages will be VERY close so you don't have to worry about one discharging lower than the other. All your esc see's is one big fat 4S lipo. If i was you, i would drive it around and just watch it. Bring it in every 5 minutes and check the voltage. When it starts to get close 12.7V i would just run it around close to you waiting for it to cut off. If you hit 12.3 and its still running then stop. I think its going to work just fine. Also, i am thinking if you are at 12.7 and you nail it, its going to cut off. Hopefully it start turning on and off on you.
Let us know what you find.
edit: i can't find a pic of a stock emaxx esc so i don't know how its wired stock. Whether the batteries terminate into one terminal or of they have seperate terminals. If seperate, you should be able to have one battery connected to one side and then measure 14.8V on the other battery plug.