Maybe he sees the cement as an ok place to go, because cement is also outside...I don't know, I'm groping! But, I know my friend's dog would pee on this green "grassy" welcome mat they had. It's a thought. I still think a crate would be a good idea. Could you get one big enough for both of them? I don't think it would be mean to have one in a crate and not the other, but, if you're concerned about that, could you put them together? And, with pee pads, once he's going on them, you could put them outdoors with you whenever you go outside to let them go to the bathroom. And, then, when he's consistently going on them, you could remove them to see if he'll hold it over night. Just a thought...