I've seen that some folks make their own Colt grips using two or three pieces of wood.
I think that they construct a wood connector piece in the middle of the grip.
I'm not sure why they choose to do it that way, maybe to reduce the level of difficulty for the layperson to construct them.
Perhaps a similar method could be used to construct 1858 grips.
Or else a one piece 1858 grip would need to be connected some where, based on the preference of the maker and the modified fit that would be desired.
For instance, if someone wanted a longer 1858 grip then being connected at the bottom of the grip frame may work. I think that a bottom connection has been tried with the Ruger Old Army grip. Not necessarily a one piece grip, but connected for the sake of lengthening it for someone with large hands.