Looks from that pic like you are talking an inch or two deep of gravel from shoulder to shoulder. You don't say you are uncomfortable riding with pillion on that sort of surface on the straights. If you're comfortable, that says plenty about your skill level, IMO. That sort of surface two-up on a big, heavy bike like a GS with 19-inch front end is tricky. You must have the "relax and let it move around" bit well worked out. But you're not comfortable in the turns. Turns you believe you would be quite comfortable in solo. It occurs to me that your intuitions might well be accurate here. That is, that there is something about the combination of weight distribution and chassis geometry two-up that is interfering with the turning process. While - possibly - enhancing the straight-line stability. One guess: perhaps two-up the bike sags more on its rear spring (even when preload is added), which would change the effective steering head angle, biasing it more towards straight line stability and less towards steering bite. Add the more rearward weight distribution (which would tend to bury the rear tyre a little deeper than the front) and it seems explicable that the front feels (indeed, is) less secure in the turns than when solo. What to do about it? I've an idea that if you can understand the problem, you'll sort out your own solution. Good luck!