Does using multiple shear pins under a limiting torque case reduce the minimum required shear pin diameter for the coupling or is the complete load distributed over the shear pins regardless of the number of shear pins that are used?
Multiple pins would share the torque if they are radialy positioned and the sleeve bore holes are machined snug.
For example in your case if we have two perpendicular pins the shear on each end is 1/4 of total shear.
In many precisision applications such as aviation or heavy construction that they require multiple fasteners, even in complex geometry like the empenage of an airplane, the load is divded between the rivets. So each rivet takes a contributory load as a fraction of the total joint load.
There is one pin in each shaft, so each pin takes the full load but in two places. This is known as double shear.
If you put 2 pins for each shaft then there would be 4 sections of pin taking the load so the pins would need re-sizing, otherwise they would be too strong and something else more expensive will break.