Depending on the species a 10 is OK for a pair.

For the better beginner species (Azeurus, Leucomelas, Auratus, some Tincs) which are also a tad bigger compared to other darts, you really shouldn't have anything under a 20gal for just a pair.

Also remember, for naturalistic set-ups the bigger the better. And I mean better in the fact that it's much easier to maintain a healthy vivarium. The smaller the tank means even the smallest problem can have a disastorous effect. Larger tanks can handle more.

Like I said, smaller tanks are a lot more work/harder to maintain than even the biggest of enclosures.