A pair of thumbs in a 12x12x12 would be a tight squeeze. Realize that some morphs are arboreal and wouldn't appreciate that space. A 12x12x18 is equivalent to a 10 gallon vert. You could get a 10 gallon for $10 at the $1/gallon sale at Petco (starts again Oct 3rd) and then spend $20 on a vert kit. Actually with that savings you could go up to a 20 gallon and vert kit it. If you're worried about budget that's what I would do. It'd be better for the frogs and you'll be more likely to get them to breed if they're happier. Then you could recoup the price of the tank.