Since you only have 2 BPs (I think), a space heater to warm the entire room is probably overkill. If you aren't opposed to higher heating bills, you can bump your house/apartment thermostat a few degrees. You can insulate three sides and 3/4 of the top of the tank with cardboard or some type of sheeting - glass does not hold heat in well (I recommend this even if you heat your house some more). If these don't do the trick, place a ceramic heat bulb or infrared bulb outside the cage and regulate it with a thermostat to get the proper ambient temps.
What size enclosure are they in, if downsizing is a 15qt? Are they housed together or separately?
I agree with the recommendations to stick with the prey that the breeders was feeding before trying to switch them to what you want to feed - baby steps.