Hello and welcome to FF! Myself thinks in the long run you would be better served by returning that enclosure and gettting two 18x18x18 ExoTerra's. If keeping, the divider will have to be a solid color so frogs do not see each other and both frogs free of any pathogen that could infect the other one. While they grow to around 3.5 in. (approximate transfer size) you can keep them in smaller critter keepers inside the larger tank since it's easier to control temps and humidity that way.
Not sure where are you getting information to make decision on a C. cornuta (Surinam Horned Frog); but those are one of the hardest to keep alive and challenge even that experienced Pacman breeders. You might want to read this article and maybe go with one of the easier Pacmans like an Ornate, Chacoan, or Fantasy. Good luck!