Why two external dryer vents?
Vents could be 


exhaust for dryers, bathroom exhaust fans, stovetop exhaust hoods; 
intake (fresh air supply) for gas- or oil-burning appliances (furnaces, water heaters, fireplaces); 
or plumbing vents. (Although plumbing vents would typically be through the roof.)


Can you follow the duct inside to see where it goes?