What kind of riser material are you using cobbow?. If it is a laminated riser, I would say you could cut it to center (leaving 3/4 or a bit less in the riser). I just checked out a 58# longbow I made recently, I cut it past center and their is 5/8" of material left side to side and it had 1 3/4" back to belly. The section had 11 lamination front to back though.
If you are going to try a double rest, would recomend keeping it thicker at the back of the bow (say 3/4") and then radius the cut to the belly coming in more (say 3/8"). I saw this on a 60s Bear once and it looked and shot quite well.