it's easy.
stack the pieces and tape them together with the ends aligned.  draw the z-splice on the top billet.  Cut them both at once.  Presto!  They fit together.  NO problem.
If the cut is a little gappy, you can steam or boil the billets and then clamp them together.  This makes the wood conform to the cut and closes the gaps.  Leave it clamped for a few days in a ventilated place so it can air out.
When fully dry, glue them together and you are good to go.