I believe they stack like this.
If a normal weapon hits a lucky/etheral one of the abilities checks first, and then if it passes that check, the other ability checks. If the weapon succeeds at passing both checks, it deals damage.
So the probability is (1/2)*(1/4) = 1/8. So a ethereal/lucky unit will only get hit by 1/8 of all the normal strikes.
Therefore, you're correct
Jazzepi