Depending on your income there may be community action programs who can help. Here in MN we have something called CEE which offers low interest loans to homeowners with lower credit scores. They also have emergency programs for those who can’t pay. The company you got a bid from should know of some resources and if they don’t, ask around at other companies. Why do you need to do both right now? Couldn’t you start with furnace only since that’s more urgent and do air later?