Vertical Tearing / Sync on tiled 4k display

I have a 4k monitor (Dell UP2414Q) at a GTX680 and it works with MST @60 under Linux. The fact that it is still treated by the Xserver/Software as 2 monitors is almost ignorable (I hacked my windowmanager to get the maximize buttons behave right), but what is still annoying is a tearing effect. It appears when something moves vertically (like e.g. scrolling text - so it happens very often), the two tiles of the display are not in sync. The right and the left half are offset, with the amount depending on the moving speed. I assume it is no monitor problem, as when I boot my machine under windows the effect is not present.

I am no expert, but a guess would be that due to the fact, that they are internally treated as two different monitors, which are not synced (and I fear it would not be solvable without modifying the xserver/driver to handle it as a single entitiy).

I just checked, when using SST (drawback are the only 30Hz) the tearing is gone.