Dual strobes do offer a variety of options that you won't have with a single strobe - just like adding a WA lens will offer options that you don't have without it
Although I see what you are trying to illustrate with the two photos above, it's kinda a misrepresentation as you have can have much more evenly lit subjects with a single strobe if it is used to create that look.
Single or dual it doesn't really matter - you need to learn how to use them. Some people do really well by jumping in with both feet and heading for duals; others find that working with and mastering a single strobe is plenty for quite a while.
Don't forget things like dual strobes are "harder" to push through the water and you can't stick your rig in areas that are really tight as effectively; but you might be able to light something that wouldn't otherwise be possible with a single. Everything in underwater photography is give and take. A single strobe will often give you a shadow where you might not want it - you need to learn how to control that to make it work for you. That might mean exaggerating the shadow, eliminating the shadow or getting the shadow to complement the rest of the image.
Shadow uses: drama, mood, knock out distracting elements or backgrounds, to make the subject pop from the frame, to concentrate attention... Just like every other compositional and capturing tool/trick, it's a matter of finding what works for you in each image. And if you want feedback from others, finding out what attracts other viewers.
One or two (or more) - you can do all sorts of creative things! Happily, there is no one way to do this whole uw photo thing
