Well,
To clarify, we can't IMMEDIATELY rule out that there is no second faction. There are these longshot alternatives:
1. Each team is even/odd meaning mafia group a can kill night one, mafia group b night 2. We've previously had a game where that was the case.
2. Mafia team A has direct kills. Mafia team B has poisons (which kill after one day)
3. Mafia team A or B chose not to kill
4. Mafia team A or B were prevented somehow.
These are all very longshot possibilities, but they should be known nonetheless.