By the way, when the yourglass spin too quickly some of the balls got shooted out of it.
Oh yes, that is a known...
feature.
I intended to fix it, but I soon discovered that a winning tactic was to simply time your rotation with your partner and empty as much of the glass as possible. Easier to trick your opponent with less sand.
By-the-book, the Objective is simply to empty one's half. What your partner receives is mere formality.

Also, the only "foolproof" solution to the issue seems to slow the game down too much.
I won, obviously. I'm good at sharing

I say, a bona-fide gentleman!