Oh boy, this morning we've got Mad-Molly-Monster-Mouth I'm thinking that dragging an old cocker spaniel around - backwards - by his tail is a step too far. Back in the playpen time do you think? Phew ...
A step too far for sure. When Tatze arrived at 10 weeks old we had a 15 year old Cavalier. Tatze was the same, so we had safe places for the older dog. How would the spaniel like to snooze in the playpen? My Gavin would happily snooze anywhere we didn't allow the pup.
I don't suggest this would work for yours, Gavin was very easy going and would snooze anywhere lol, but this was our arrangement, outside and inside. He did let Tatze near when she was behaving - as the last photo shows! I think a safe place for the older dog is the answer - it mean you can train the puppy without constantly feeling you have to confine them.
you can always get in too. We used to make barrcades for Doug with the soft funishings. Moo really used to go mad at him and drive him to utter exhaustion but one look from Doug and Rory would behave.