My first thoughts are to be careful with using your furnace exhaust.
If you don’t get it right, whenever the furnace is running you could get carbon monoxide being exhausted into your basement if you are pushing forced air into the ducting. An HVAC pro would have better advice, of course.
Although, that second opening that is below the furnace exhaust appears unused. Might want to investigate that, too. Two flues out the chimney?
edit: opps, nevermind. You were talking about the second exhaust in the first place. Sorry 'bout that. Reading too fast.