Reply by Sawkerf

The trick here is to NOT use the part of your biscuit joiner that controls the depth of the slot.

Mark the center of your slot on each piece. Sit your biscuit joiner plate on a flat surface and make your first slot. Sit the other piece of wood on its edge and make another slot. Since you’re registering from the same surface, the slots should align just right.

-- Adversity doesn't build character...................it reveals it.