Two external keyboards, one for right hand, one for left hand, modifiers do not work

I am in my bed. MacBook (Yosemite 10.10.2) is on my chest.

I have one external keyboard (with trackpoint) for my left hand and another external keyboard for my right hand.

This would be awesome if it would work.

But the problem is that when I hold shift on the left keyboard and press P on the right keyboard then Mac writes a p instead of a capital P.

If I hold shift on the right keyboard and then p on the same keyboard then Mac writes a capital P as it should.

The same problem happens with all modifier keys:

If I try to use one modifier key on one keyboard and the to-be-modified key on the other keyboard then the result is simply the to-be-modified key unmodified.

Any idea what is causing this behaviour ?

How can one fix this ?