Doing drywall panels on ceiling share joists at edges?
The edges of drywall must share the stud/joist. If they float, they'll sag, no matter how much tape and mud you put on them. Especially on ceilings where gravity will be the enemy.
If you're having serious issues getting the screws into the edges of the sheets on only 1/2 the joist width, you could add a 2x4 nailer to pick up the edge of the next sheet, but that's a last resort.