Everybody,
thanks to
gifu's description, I found the issue : it happens when switching from the sliding sliced backhand to the sliding regular backhand (which is currently all the time, coz of another bug

) ; in such a case, the anim default to the 1-handed backhand, even if your player is 2-handed ; as the 1-handed animation isn't loaded, the state may not be the same on both PCs and thus it desyncs.
Playing with 1-handed backhand should avoid the desyncs. And if both users switch to 1-hand backhand and back to 2-hand backhand for their 1st & 2nd player (in Training Club -> Character Sheet), it might prevent the desyncs as well.
I'll update the game as soon as the exhibition tournament is done (ie: when I know if we switch to 83% skill average for the Fair & Realistic modes).
There's no need to send me Bug Reports about this issue anymore.