I have a modded to death Jagmaster with p90's. thing is buzzing....however with the toggle in the middle it is dead quite. would shielding get rid if the buzz?
p90's are single coil pickups, so they're going to pickup what any other single coil does. Shielding can help, but won't eliminate it.
The reason they're silent with both on is because one of them is reverse wound.
JJuran got it on the money. I would also check to make sure all your grounds are in the right place, bridge grounded, pots grounded together, every ground going to a single point, etc.