when I have to share a bolt between fenders and rack, I run the bolt in from the inside (hub side) on the non-drive side. Then I put the rack on, then a nut, then the fenders, then another nut. Washers are never a bad idea, and the outer nut can be a nylock nut as suggested above. On the drive side, I run the bolt in from the outside, but I use two nuts, one to hold the rack on, and the other to tighten down the fender. I think my method might require 30mm bolts depending on the fenders and rack.