Sometimes, we have to look at the maker and/or the school. Armstrong flintlocks, and guns made with the percussion system, all have two bolt locks. So do most of the other rifles made in that region. Ditto York, although you will see some single bolt guns of either system. The Bedford school, almost solidly percussion, very nearly always used two bolt plates. I wouldn't want one that did not.
You see it in other schools as well, e.g. Lancaster. I have an original percussion M. Fordney rifle of 1840 with a two bolt plate. Gibbs and other makers used two bolts to secure the lock in many of their guns.
Tim mentioned an early Dickert that had a one bolt plate; when I saw it my reaction was the same as his. Something seemed to be wrong. But, the English were said to be using single bolt locks as early as the mid 1700s, and I happen to know who found the rifle in the south, and when. It is quite good and qualifies as a 'Deckard Rifle', by which they were sometimes called, down there. It is shown in the second KRA book (edited by James Johnston with the gray covers): "The Kentucky Rifle, 1750- to 1850." It may be that Dickert tried it after having heard of its use, made a few, and abandoned the idea. This one is the only one I have ever seen. As you look at his work and its variations, you can see that he seems to have been in favor of trying new ideas and styles so this would not be surprising.
The single bolt lock plate has in its favor the fact that the front bolt does not interfere with the ramrod channel as is often the case with the two bolt. I have seen some rifles with the front bolt nearly filed half in two to make way for the ramrod. A single would likely have been less expensive and also less work.
So, there must have been many factors modifying gun building decisions way back then that escape us today.
Ideas, anyone?
Dick