Research, research, research….
 
There are many reasons why mixing two random species of dart frogs is a bad idea… But that does not mean that in every situation mixing is a bad idea…
 
I oppose hybridization, as most people do… but as you said you do not wish to breed, so this is a moot point.
 
Sticking 2 frogs of two species in a fairly bare 30~40 gal tank is asking for trouble… but putting two males of one species and two males of another species, taking the time to research the species so you choose compatible species, taking time to research tank planning to create décor that will support 4 frogs, etc, etc… will allow a 40 gal tank to hold 4 frogs…
 
If you just slap it together and pick species based on appearance, you’ll screw yourself… If you take the time to educate yourself on the animals you wish to keep and keep them based on their needs, you’ll enjoy some amazing little creatures…