How many Pork Shoulders?

"I would like to smoke 4 pork shoulders about 6 lbs. a piece in my Weber Smokey Mountian Cooker. Can I cook them all at the same time? Can I please get some advice on the best cut of meat, approximate time ( 1 six pound boston butt took me 16 hours). What about the meat on the upper grill dripping on the lower grill? I used advice found here to smoke my first shoulder last month and it turned out fantastic."