You could just add an extra nurse or detective. Since your rules say they can choose to use their action every two nights, they would just agree when to use their actions, and in the detectives' case, they would agree who to investigate, just like the mafia members agree who they should kill.
And SG, it's actually very balanced. The mafia members are guaranteed at least one kills before they can lose, and the chance of one of them being killed in the early rounds is low because they will vote for the same person to be lynched during the day, and just add their votes to someone else that has a lot of votes. And if one of them is clearly going to be lynched no matter what, the other mafia member votes to lynch them as well so they won't be suspected as the other mafia member. Even if it's five people against one mafia member, the mafia member can still win. I would know, I did it the first time I played :-D I even voted to lynch my partner in crime to throw off suspicion. Two against ten is quite fair (or two against eleven).