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.