Usually the two stems are two individual plants growing from two seeds that were stuck together when planted. Some people would kill the weaker plant. Some would try to separate them so they would have two plants, with the possibility of injuring both if you didn't do it successfully. I suppose you can plant the two plants together, but since their roots would be fighting for nutrients from the same soil volume, and their leaves would be fighting for sunlight from the same above ground area, I would guess the two plants together would probably produce as much as one plant in the same area, if they didn't starve each other out.
I never leave two plants in the pots I sell, but I can give them individual attention that someone mass producing plants couldn't.