How many dividers are you going to put in? The fish should have plenty of horizontal swimming space if you only do one or two. I've heard of people keeping bettas in deeper tanks than that, so as long as they're healthy (no buoyancy issues) and they have enough horizontal space, I'd say it looks fine. If you're still unsure, you could do what Canuck Fins suggested and partially fill it, as long as that works with whatever filter you have/get.
That's a great-looking tank, and I'm sure your bettas would love to have all that space!