Thanks for the advice: I think I had better give the two leads at-the-same-time method a go. Funnily enough, it is actually the older, bigger dog who pulls in a tandem situation. One thing I have been doing - and maybe it is a giant no no, I'm not sure - is putting the lead behind my back and kind of bracing it to keep them in line, especially down hills. I usually only do this, though, when I am tired of heeling and correcting and my left arm is a bit worn out. They do get walked separately when my husband accompanies us, which is quite often, otherwise I don't think I have the physical strength (or time!) to go on our big, long, up hill down hill walk twice a day! Maybe I'm just too fussy wanting them to heel perfectly?