The only way I've seen to be able to use an award certificate is to choose to pay with points and then it will let you. However, if you only have 1 certificate then it seems that you have to have points or buy them to pay for the other night.
If I want to use a certificate along with a paid night do I just have to do 2 1 night reservations? 1 paying with points that will take the certificate and then one paying cash? Then I suppose I would most likely have to switch rooms, but I guess that's better than switching hotels (which I'm also considering 2 nights each at 2 different hotels).