You just have decide how many stops to underexpose for the moonlight and how many stops to overexpose for the lightning. It’s a creative decision based on how well do you want to see things before and during the flash. You’re shooting film so you have a lot of headroom to overexpose the flash while still holding detail. You could, for example, have the moonlight be 2-stops underexposed and the lightning flash be 3-stops overexposed, which would be pretty dramatic. But the lightning could also be less hot - in reality how bright it is depends on how far away and how intense the flash is. You could have a few dimmer flashes that are only at key level and then a big one. If you need to see more action in the room before the flashes, you might decide that the moonlight is only 1.5-stops under (and that also depends on if the moonlight is a side light, frontal, or a backlight.)