Well I dual grip similarly but a bit differently than what has been said. Other than slight adjustments for the specific gun model/shape, I shoot pretty much any hand gun the same basic way.
The strong hand is more or less a given. though I would add to keep the grip strength on the lighter side. Something a little shy of the gun being able to move around in your grip. Firm, but not too tight. Too tight will pull your rounds to your strong side.
As for the weak side hand, much like contender said, I run my hand forward and warp my fingers around my strong hand. On my BH, the first knuckle of my index finger of my weak hand presses against the front of the trigger guard, finger up against the bottom of the frame. If I have my strong hand trigger finger laying along the trigger guard, the point of it will just touch the pad of my weak hand index finger. The middle finger of my weak hand runs around the lower front of the trigger guard and wraps up onto my strong hand fingers. My third weak hand finger wraps around the third strong hand finger and same with the pinky.
My weak hand thumb stays raised and just to the side. My palm will be slightly off my strong hand. There are a few reasons for this. 1) It encourages better balance, 2) It reduces "over-gripping" 3) The off-hand thumb is the best way to cock a SA (especially for faster firing), and lastly 4) if you ever shoot semi's you won't wrap your thumb around more than once. Helluva gouge that slide makes!
The grip should be relaxed, but controlled, natural but trained. I subscribe to the push-pull method, albeit judiciously. Push slightly forward with strong hand and pull less slightly backward with the weak. It's a wonderfully stabilizing grip. Just mind your pressures so you don't tweak the gun to one side or the other.
Hell, I reckon a few pictures would show it much better, I'll see if I can get a few in the next day or so if anyone cares.
Rodfac's grip, IMHO, is much of a target-shooting type grip and no doubt well suited for it. I come from a more "tactical" type background and my grip style reflects it.
Now, as to the question of how they did it "back in the day," I'm surely no expert on the matter but if you look at PERIOD paintings and illustrations (meaning those made in that era, you rarely see two-handed grips or even extended arms for that matter. It's almost always one-handed, cock and rather tight to the face by today's standards. Often with the head leaning back. I reckon dualists being something of an exception. remember, the average gun-toter didn't really have anyone teaching him "proper" techniques. They would do what they thought seemed fairly natural.
Just my 2 bucks.