I think it depends on total weight. I have my bike set up for me at 245 lbs. With a total weight of something like 400 lbs., with luggage and a reasonably sized 2nd rider, I am going to be pushing the max load limits. I have bottomed out, riding 2 up, entering into a turn aggressively. I can't help but think that my shocks aren't real happy with e at that point. Before I got my girl her Busa we used to ride 2 up often. She is like 140ish at 5'9". I actually got a larger capacity spring for the rear. But when she wasn't on, it was like riding a bronco over the bumps.
I think if you are say 170, you should have plenty of room for the suspension to do its job with a second rider.