The ghetto-fabulous way I do it is hit the shared blizzard key, then rapidly flip around between the monitors targeting and casting. Since it's just a mouse position and click it happens pretty fast. With improved blizzard you can still chew through mobs pretty good even though the cast isn't simultaneous. Multibox's mouse sharing doesnt work for me at all in terms of identical cursor positioning. Haven't been worried about doing this enough to invest in a hardware solution.