Doesn't matter. I have a two bike Sarris Bones and I have the same problem. The only thing I have seen so far that keeps bikes from bumping is a three bike rack but only two bikes loaded.

Two of my buddies have 3 bike racks, one a Sarris and the other a good brand I can't remember right now. Both work great with 2 bikes but have problems when 3 bikes are loaded.

I have found the best thing to do is to tie them tightly together, so they move together. Pad between bikes where needed.