I remember trying to ride with Jasper (springer) in tow, and to be honest I think you're doing the same as I did - rushing things a bit! It does look terribly romantic, seeing obedient dogs following horses, but I also remember having my heart in my mouth (or mi amino in mi nase as we said in Latin!) as there's so much to go wrong and I didn't go near any roads!
I was going to say, shouldn't there be a 'T', but of course on opening your thread you're being norty and changing acronyms!
Is there any way And or someone else could walk alongside you, to the left of Jak, with Sam? Or maybe you could lead Jak (to the right), with Sam on your left, as part of training? One idea in my mind is using a target stick to walk him with (minus Jak!), so that he follows that, regardless of which side he's on. Maybe use a schooling whip as the 'target' (so that it's transerable to ridden work, later). Those are my bumbling thoughts!
