Which model would you be thinking about, the 19 or 25 litre one? Personally, I wouldn't keep both a betta and frogs in the 19 litre, and I'd be concerned that the 25 litre one is too tall at 41cm (16 inches). Frogs breathe air, and they can struggle to reach the surface if the water is more than 12, maybe 14, inches deep. And with the open top, the betta could well jump out. I've had experience of a female betta that regularly used to jump out of her tank whenever I opened the flap to feed them. I'm not sure if the frogs could get out as my tank has a lid so I've not faced that problem.
There's also the problem of feeding the frogs. My last betta didn't like frog food, so there wasn't a problem. But my current betta loves it. I've had to construct a cave with tiny frog sized holes to put the food in. You'd be amazed at the size of holes a betta can get through when he sees food on the other side.