Roy would be the best one to answer that I think so hopefully he will see the request. I received an unknown female that I suspected was post hatch when she arrived and I never saw her again after she was released to the aquarium. She was the size of a large merc but her skin was smooth and she did not have red in her dark color. I suspect she was joubini. She matched well with Norman's joubini photo and was caught in the Keys where they are known to inhabit but are rarely seen in the trade.
Why are you anxious to know? There would be no difference in their care requirements and not likely in their behavior.
One of the mian differences between O. mercatoris and O. joubini is the egg size. In the case of O. joubini, the females lay small eggs which hatch into planktonic young. O. mercatoris females lay large eggs, which hatch into bottom dwelling young. It's possible to raise these young, which a number of our Tonmo members have done. You can read their accounts in the Journals and Photos forum.
And usually, when someone tells you you're getting a joubini, it turns out to be mercatoris!
Do you have a supplier in mind? I only ask because I am aware that most collectors in the keys think the dwarf octos they find and sell are joubini and advertise them this way. The collector I work with was one of these and would not accept the correction on my word only but knew that I was trying to learn to care for and identify octopuses so he sent one to an expert. I'll avoid names here but this expert and recent staff member confirmed the mercatoris ID (in the meantime I had raised the large egg babies, also confirming my call). Ken's site is the only one I know of to properly identify the dwarfs caught in the Keys (he only has them on occassion and obtains them in trade from other collectors) as mercatoris.
If your supplier is local and does not know the origin of the animal, there is a slight possibility of joubini but it is unlikely.
Dan knows his little guys are mercs (he is not in the Keys). He has not posted in a long time and I didn't know if he still offered them. If I recall correctly, they are usually brought in with the aquacultured LR.