i use 8 guage on my subs, if you only have one pos and neg terminal on each coil then you have to solder the wires from the second coil to the ones on the first coil with enough extra lenght to get the firstwire in the terminal
Your best bet is to use the largest gauge you can. I am not sure if you have spring-post connections, or if you are going to be soldering, or using connectors of some sort, but always go with the largest gauge wire you can. i have seen people with 4 and 6 gauge wire for their subs, but 8-10 gauge is just fine. even 12 works, but as i said before, use the largest gauge you can.