There are rules on how much air louver area you need based on how many BTU the heating devices utilize. My guess is that if you can supply that amount of makeup air, you should be okay. But, when both are on, that amount of air is substantial. Have you considered a way to let it get air from outside verses pulling it through cracks in the house? All that air that goes out requires air coming in, and you pay to condition it: cooling in the summer and heating in the winter...much more efficient letting it pull it from outside on its own, dedicated path.