I've been asked to make a 40-45# 31" draw longbow which makes a bit of a change as I've been doing warbows lately. I have some billets from 2012 which aren't hefty enough for a warbow but they should be fine for this bow.
The problem with splicing billets is they are rarely perfectly even, so the back surface of each billet doesn't align nicely (see red arrows on top pic). The front edge of the splice is low. The back edge was high but has been rasped down exposing a little heartwood.. To over come this I'm rasping the back to a flat, concave, even surface and then overlaying a curved slice of sapwood (from the same wood as one of the billets). This has the added advantage of strengthening the back at the splice. I've used this technique before when I had a splice fail and I feel it's a sound belt and braces solution where splices are done on rather skinny wood.
The pictures pretty much tell the story, and you can see the quality of the splice where it has been rasped down, there are no gaps or excessive glue line. Note in the bottom left pic the pencil mark for alignment of the back overlay when I glue it down. I apply the glue, secure it in position with masking tape, then bind it with a couple of layers of rubber strapping to hold it tight while the glue cures. I use Resintite glue.
You can see in one of the pics, the right hand billet isn't as thick as the left. I may add a belly patch in the grip area, this will be tillered more Victorian style rather than Warbow style, so some extra thickness at the grip will look right.
The sapwood is just about the right thickness on the left limb and I'm hoping to be able to leave the back of the bow in it's natural state, by just carefully removing the bark. This may not be possible on the right limb which has slightly thicker sapwood. Once again this is the difference between theory and practice. In theory you have two perfectly matched billets split from the same large diameter Yew trunk with 3/16" of sapwood and evenly curved backs..... yeah in my dreams.
Although oddly, in my dreams I never actually get to cut the Yew, I'm always searching for some tree which I can't find!