They don't necessarily have to be the same species to have compatable DNA as has been seen in other characters.
- Spock was half-human and half-vulcan
- B'Elanna Torres from Voyager was half-human and half-klingon
- Deanna Troi from TNG was half-human and half-betazoid
There is also an article on Memory Alpha that details other Hybrid characters that have been seen.
Taken from the Memory Alpha article
Because of the common genetic ancestry of most of the species of the Milky Way Galaxy by way of the ancient humanoids, many species are able to interbreed with or without the help of genetic technology. In fact, Humans and Vulcans are quite similar. (TNG: "The Chase"; ENT: "Demons")
In some cultures, children who are born as a result of interspecific relations are rejected by the society of both races. For example, Bajoran / Cardassian hybrids were often rejected by both Bajorans and Cardassians. (DS9: "Indiscretion", "Return to Grace")
This would suggest to me that they are separate races and able to conceive either naturally or with some assistance from medical technology.