Actually, it wasn't that hard to distinguish between the two strains once they were a couple of days old (which is how long I puzzled over that two-headed plant before figuring out what had happened). The other plants (Blue Mystics, Indica-dominant) have their own distinctive look, so I killed the one that looked like the other Blue Mystics, and the freebie seed (Sativa-dominant Durban Poison x Skunk #1) took off growing. The stems of the Blue Mystics when they were really young were more purple and almost semi-transparent, just totally different from the freebie seed plant.

Just planted five Nirvana Skunk #1's, also freebie seeds. I don't hold out a lot of hope that many will germinate (they're probably old and not very viable, which is why they give them away), but eh, maybe some will pop up. I'm giving them a full week this time; I learned my lesson!