Half a FN wouldn't be enough, unless you mean one unit of a double FN, in which case yes one unit is big enough for two chins, but doesn't give them much room to get away from each other. However depending on how old they are keep in mind that kits under about 4 months might still be small enough to escape a FN, or could get stuck in the bars. You can either get a CN instead or just cover the cage in hardware cloth until they are bigger.