I don't have the manual for that specific backplane but the numbering shown on yours is weird. It does seem like the board designer wanted to group the two ports for "J49" together as a functional unit but you should be able to treat J49 as either an 8-lane wideport (two SFF8643's) or two 4-lane jobbies. Same for J50. The important thing is that there cannot be loops in the topology. In your righthand drawing, you cannot add a cable from the 24-expander to the 12-expander because it would create a loop in the topology.