It took me a good three years to hook a set with the dolly attached to the front box. But then, I've always had a road job, so the vast majority of the time, I never had to do it. Mine were almost always stacked, or the dolly was spotted in front of the back box. So I never really had to do it. Hence, the three years. But as everyone says, and everyone told me, one day it just clicks for you. For me, it was just getting over the mental block of trying it, because when you first try to get it, as most of you can confirm, it seems IMPOSSIBLE.
But it comes. Smash cut--for me--Monday morning, when my back box was between two long boxes with tractors attached, and my trailer was about ten feet back between those two tractors. I drove my front box with dolly in tow, to my spot, and backed up. I made several attempts, but realized I needed to drive back around and get a better angle. Next time, it took my three backs and pull ups before I hooked them.
In the past, I might have tried to back up close and drop the dolly, and push it up to the back box, then back the front box to the dolly, then hook up. But with time, you say, "Screw that. Too much work."
I had an old grizzly veteran try teaching my at first, but after watching me try it, he came out and the first words he ever said to me, were, "You can't back that dolly worth a friend$ck." He laughs whenever I remind him of that, and always reminds me, "But you can now, RIGHT?"