My chihuahua puppy keeps peeing constantly?

I have 2 6 month chihuahua male puppies. They usually hold it up to 8 hours when their in there kennel. But one of them keeps peeing constantly. I mean like I can't really take him out because when he pees it's unpredictable. So, what can i do?My other dog used to do this but he is potty trained. And my other one after a walk he pees and poops sometimes. Not all the times. So what can i do about this?