A dog with a bladder the size of a pea

One of my two dogs is having an issue with peeing. I have taken her to the vet and all tests have come back negative, so it is believed to be a behavioral problem.
   
She needs to go out and pee approximately every hour or two during the day and evening (there is usually someone home during the day and we are both ususally home during the evening). There are times she will be laying there and small amounts will just come out. If we go out and leave the two of them alone for 2-8 hours, we never find any accidents when we get home. It is like she has the bladder the size of a pea (no punn intended).
   
Our other dog never has an accident and can hold it for 8-12 hours.
   
Any suggestions?