I ordered a painless dual battery kit and in the instructions it says to run a wire from the starter solenoid, to the (kits included) solenoid, and then from that same terminal to the battery. I was wondering, due to lazyness since it'd all be connected anyway, if I could get the same effect, by just running a battery wire, off of my battery post to the solenoid. And just leave the battery cable that's on it, plugged in. Which does go to the starter solenoid correct? Basically the instructions suggest.

Starter Solenoid -> Terminal on Solenoid -> battery wire from same Terminal to battery.

Would that be the same thing basically as

Starter Solenoid -> battery positive -> battery postive to Solenoid.

Also it suggest upgrading to 2 gauge wire. Is that really necessary?

Hope that wasn't too hard to follow.