[Fred T]

dmarty is on the right track. The cracks are only in the surface, a result of concrete expanding and contracting. It used to be standard for a floor to crack, which is why they cut the surface now.

Personally, I am not a fan of two post lifts, but they have their advantages. Big thing is to balance the load. It's the unbalanced load that tears the bolts out of the floor. With a good mix and the right steel in a 6" slab you won't have any problems. I would stay away from the cracks, if possible, but wouldn't be too concerned if there was a crack between the posts.