They said I was mad, MAD!
Cargo almost certainly could be stacked on both sides of a barge -- but there's clearly a hard weight limit imposed by arcanobabble and games mechanics, which is likely more of a concern than available space. Stacking all your cargo on one side of the hull is useful not just for landing purposes but also for day-to-day operations (diving through the gravity plane every time you want to check what's strapped to the underhull is potentially hazardous) and potentially for last-ditch defense (pitching so that the keel of your ship is facing towards the enemy, rather than squishy cargo and crew -- the shell of a cargo barge may be fragile, but it's better than nothing).Am I correct in remembering that the gravity line is a plane, above which things fall down, below which things fall up?
If so, why are we not stacking cargo on both sides of the barge? (Well it's going to be a bit awkward if you land like that, I guess.)