A 30 long is a great size for D. tinctorious. Frankly, you could get away with a 20L or a 15 for a pair too. Mixing Azureus and Cobalts would not be a great idea. They're both the same species and so this presents even more problems than if you were mixing different species. You would risk creating a funky hybrid by putting them together, which is generally frowned upon (although it is not nearly as serious of a problem as most people make it out to be). A more serious problem is that all of the Tinctorious frogs can be quite competitive with one another and don't make good group frogs. Most people keep them as pairs or reverse trios (2 males to one female). In particular the females will compete with each other quite violently. This would be exacerbated in a mixed tank with Cobalts and Azureus because they are the same species.

One possibility would be to split that 30L in half with a piece of glass and create a tank that appears continuous but is in fact separated. Each side would be large enough for a pair or a reverse trio if they're well planted.

Cheers!
Alex