Its possible, I have done it many times, with boards and billets. I use a japanese saw, but a good western rip tenon saw with more than 4" depth of cut would be good too.
The secret is in the marking out. I leave the back crowned on the billet, and carefully square up the sides and belly with planes, rasps, etc.
I mark the splice on the back, and then project with a square onto the belly side. This is important, because with a bandsaw you can get away with just marking the back, as a well set up saw will cut square. With a handsaw cutting from just one side on a thick bit of timber it is incredibly hard to insure square cuts.
Instead start on the back, top, follow the line down at an angle so you are not cutting the belly side blind. Then flip the sides around and do the belly the same way. Lines on both sides, and the top are now cut with a hidden pyramid of wood still in the centre. Now saw down , the hidden pyramid,using the cuts to guide the saw, then the waste will pop out.
If your splice turns out too tight, you will have to geta thin rasp or file and work the splice, checking the fit, holding it up to the light to see gaps.
Don't force the billets together if they aren't ready as this can cause a split. Putting a clamp at the fades when tapping the billets home will help prevent this happening.
Use a good quality gap filling glue like epoxy or urac.
Hamish.
