Wow & shift: strange behaviour on some keys - solved

In order distinguish casting spells between my main and slave toons in Wow i mostly use the shift button.
Example: "C" would fire wand on main and "shift+C" would fire from slave.
With most buttons on my azerty-keyboard this works fine.
But NOT with Z-E-R-T-Q- and D (i.e, your basic wasd-keys and some others from the top row).

When i hold shift on those keys, Multibox broadcasts the "shift-key" correctly, BUT the normal key is also activated on main toon.
Making it impossible to for example turn only the slave toon with "shift-D".

Any workarounds?

edit: the normal key is not broadcast, but is "pressed" on the main.
So Multibox is working fine.

Update: Problem solved:

Did some more testing.
Apparently this has nothing to do with multibox nor dual-boxing.
(updated threat-titel already)

all i had to do was assign shift-W/A/S/D/etc... to empty actionbar-slots on main.

Problem solved.