Yes--it's all handled with paper, so just bring your one certificate with you and the rental agent will take one day's charge off, leaving you to pay the other day.

The certificates state you must put coupon code "BCFD" in the promo box, but as far as we've ever been able to tell, that doesn't actually do anything as far as discounting the rate/removing a day's charge or anything and isn't even necessary (they'll take the certificate even if you don't do that).