I was having problems toilet training Indy, since we got a crate, she's come on leaps and bounds, (sorry, you don't want Archie to do that, but you know what I mean).
We tried puppy pads, but they were hit and miss, much more miss than hit actually, I've done away with them completely now, we're still having the odd accident in the house, which is mainly a sneaky wee, since tuesday, only 1 poo and that was OH's fault because he didn't follow the routine.
Could there be a bitch in heat nearby which your other dog's picking up on and Archie's picking up there's something going on which he's no way of understanding?