The reason for that in FDL was because each faction was of equal size.
The other thing you could do is go with the normal mafia ratios of 1/3 or 1/4 of the game, and then make two mafias from that. For example, a 30 player game with a mafia of 1/3 would have two mafia with five people in it each. Keep in mind that this doesn't account for third party. So in that case, you could have four mafia in each plus the cultist.
If you're worried about too many people dying at once, you could make a role to balance it. In Faes & Legs, I have a Valkyrie role that makes it so that, so long as she is alive, only one person may die per night. But, at the same time, she can only successfully prevent more than one person from dying five times total, then she achieves a separate victory and the rampant killing commences, lol.