what exactly is the point of dual woofers of the same size? is it just to give the ability to go louder due to pushing more air or is there some other purpose?
It would depend on the specific design. For instance, the second could actually be a passive radiator, not a woofer.
On the other hand, it’s pretty common these days for the two woofers to operate over a different frequency range. Typically, one woofer operates full-range, while the second operates from say, 200Hz and down. This would presumably tend to increase midrange clarity and bass capability at the same time.
The textbook answer is a second subwoofer, positioned next to the first, adds around 6db of output and helps smoothen out frequency response. I use 2, and when I added the 2nd, bass from all sources is MUCH more detailed. Works best when the subwoofers are the same model or at least have the same size driver.
Pity. I have a 12" Axiom which I bought to replace an 8" Merak Subwoofer a couple months back. If only I could get that smaller one to work I'd like to try the both of them together.
you answered your question in your initial post. it's to provide more output of those frequencies. it also increases the power-handling capability of the speaker.
and wayne correctly added that sometimes they're not crossed-over identically; ie. 2-and-a-half-way speakers.