There are too many variables to try to do this sight unseen. You have a bid already, go to the local guys and get some ball park bids from them after showing them the house or a picture. View the house again and get some pictures of the furnaces for them if nothing else. Brand, efficiency(80%or90%), SEER rating(10,12,14,or16), including an A/C and one or both coils, electronic air cleaners, and amount of duct work all weigh in on this. Figure a probable minimum of $3000 per furnace or more if you are adding A/C. Why does it have two anyway? One big one in the basement should handle 3000 sq.ft.