I'm not sure what you're trying to do here. Are you simply trying to vent the basement and such? Or are you trying to feed more/better air to your furnaces?
This is not an area of expertise for me but I think what you're asking is how to get better air to your furnaces. You may want to look into using air DIRECTLY from outside to feed the furnace(s). This way, it doesn't draw air so much from the inside of the house. I think you need to contact a professional for you to better explain your current situation and to explain what you'd like to accomplish. Your unit may, or may not, be able to operate as I suggest.
If this doesn't 'help' you, post back with more details. My best to ya and hope this helps.