no.... 2 * 8 does not make an 4 GA wire....
4 * 8 makes an 4 GA wire....
that's the same as an square 4*4 foot box takes 4 little boxes of 2 * 2 foot...got it ?....
very seldom i actually use anything smaller then 6ga...depending what is gonna be build in and if it later on is going to be expanded to more woofers...you can better spend some more money and lay an 4 Ga wire , atleast you don't have to take it back out and lay another wire when you are going to upgrade....plus not to mention it would then at the end cost you money twice...
i most of the times if i have an dua amp setup is get and 4 ga wire from the front to the back then an splitter from 1 2GA --> 4 * 8GA or something like that....
in this thing it's better to be safe then sorry.
ooh and for those wondering about speaker wire, one guy told me this once years and years ago when hooking up an woofer to an amp.
get you an bucket of water (wich would be the signal from the amp) and get 2 pipes , 1 1ft diameter and the other 1 " in diameter, then pour the water through the pipes....which pipe do you think will run faster

...it's the same for speaker wire...
heh i always tell this story to people who ask me about this, i think this was one of those lessons from an old guy that always stayed with me...