Heating can be tricky since when you add a bulb, especially as strong as a 50watt it's going to zap a bunch of your humidity. If you have the money, I'd recommend getting large heat mats for each tank and putting them on the bottom. They do help and the heat rises up into the tank without compromising your humidity. So look at the sizes on them and measure your tanks. If in doubt, probably better to go with the bigger one than the smaller one.
In the mean time I'd say go ahead and use the bulb but make sure the rest of the top is mostly covered. I use plexiglass or saran wrap and mist regularly. I use 25watt bulbs if I need to, but that depends on how cold your place gets.
I also have some handwarmers in a closet in case we lose power. Good to keep in the winter.