This was a first for me, and I saw it this weekend on the Iron Range.  State Patrol cars came down the highway and were actually driving in the opposite lane, forcing the traffic off the road completely.  Then, the semi trailers appeared driving side by side, on the back was a large piece of mining equipment straddling both trailers.

From what I learned, the trucks aren't connected at all, and the drivers have to synchronize shifting, braking, acceleration, and steering.  It's quite impressive!