Dave,
I've seen several Virginia and NC made rifles with double dovetailed front sights and at least one SC rifle with a triple dovetail.
The rifle you posted pictures of is different. I think the two dovetails are too far apart to have been used on a single sight blade. Obviously that is just an opinion but they are much farther apart than any I've seen.
That leaves a real question as to what was going on with your rifle. We know that sights were often moved as the owners vision changed. Usually the hind sight is moved farther down the barrel as near vision failed. Maybe the owner of your rifle had the fore sight moved for a similar reason or to get the absolutely longest sight radius. Wasthe hind sight also moved?
Just when you think you've seen everything something like this turns up!
Gary