When I need to cut biscuit slots into the ends of a 45° bevel joint, I clamp the two workpieces together against their inside faces. The beveled ends form a 90° “V,” which creates a secure notch for bracing my biscuit joiner to cut the required slots.
This setup also allows me to cut slots into both sides of the joint by simply turning the tool around in the notch. And, if I want to cut a double-slotted joint, I set a spacer against one of the beveled ends to move the cutter up for making the second slot.
– Serge Duclos