check out this website http://www.colomar.com/Shavano/4x12wiring.html
scroll down almost to the bottom of the page and there is a section that says "cable connections" and it shows two different ways to run multiple speaker cabs off of one tap. you can use the first set-up if the back of your cabs have two jacks. the second one you'd have to build a small patch box.
As per your first post, i'd strongly advise against running a y-cable from the speaker output. i'm pretty sure that i've read that you can't really do that.
by the way, how many ohms is our cab? you also need to make sure that both cabs have the same impedence.