Gamma you are exactly right. You cannot use both at the same time. But you do mount both at the same location (same groove in the bushings) . Notice a couple pictures up that the outer grooves in the docking points (2) on the left side are about 3/8th " longer than on the right side. This is so that the saddlebag sticks out a bit farther to clear the side of the trunk. Otherwise, it is so close clearance that the bag will not fit in the grooves, or docking points. So, you must effectively extend them to gain about 1/4 " clearance. Now, as it takes a little time to switch bushings, if I am going to run with the trunk and then go with the sissy bar shortly after, I will just leave the extender bushings in place. You really don't notice the extra space between bags and the frame that much. It is only the outer bushing you change. The inner bushings (4 all around) always stay in place and their groove is where the sissy bar/trunk will locate. Make sense??? (the) Porkster.