I don't recall if the 1st and 2nd cross sill lengths are the same or not (I think they may be identical), but I did verify with Mar-K that the 2nd cross sill is not bolted in. See the last line of paragraph 17. It just sits on the frame with the pad. I was in denial that this would work and the pad would work loose, but everything has stayed together so far.
Not sure if this was your question or not.
Mark-K installation guide
