As the tiles said it seems that the dual Weapon Mastery don't apply at all when doing a full attack with a proper weapon. In the exemple below I try with an electrostaff and add both Dual Weapon Matery I and II but was still at a -10 penalty on my attack instead of -2. I did the test with a double blade lightsaber to see if it would change and got the same result.
As the tiles said it seems that the dual Weapon Mastery don't apply at all when doing a full attack with a proper weapon. In the exemple below I try with an electrostaff and add both Dual Weapon Matery I and II but was still at a -10 penalty on my attack instead of -2. I did the test with a double blade lightsaber to see if it would change and got the same result.