I have a 2-piece set for an SAA. They need judiciously fitted.
With some Patience and a little talent in the color department the screw holes could be filled with a bedding epoxy to match the rest of the grip and make them 1-piece.
Recognize that horn is hollow with a bone center. It would be very rare if ever to find a piece of horn thick enough for one piece grips. Contact a supplier of horns grips and ask them if they'll ship you a pair before drilling the hole. Then shape a center piece of wood and glue the grips to it. Most one piece grips are made that way, although of course you can get one piece walnuts from SAA specialists like Dave Lanara, member of this forum.