Differences in the induction track and cylinders can cause bias - see photos which are from The Book of BSA Twins by W C Haycraft Pitmans Motorcyclist Library series.
It is not unusual to find that mag points gap is different between the 2 cylinders, this can be caused by the manufacturing tolerance, photo 3 from Lucas service bulletin. It is also not unusual to find that the timing is slightly different between the cylinders, could be due to bad assembly, worn or badly shimmed bearings, worn cam ring , misalignment of armature etc etc. If too far out mag may need attention.
I would check all the basics of carb including carb to head joint o ring and gaskets, points, plugs and see where it takes you.