Most places do not allow ball to ball towing so that example is only good far a small portion of people. I tow tandem with my 5th wheel as its the only legal way here
Over half the states that allow doubles actually allow ball on ball (typically TT+1) but technically any GN setup with a double tow is ball on ball.
Technically Colorado doesn't allow ball on ball so although I'm already skirting the length limits here and in most states we travel, I'm not using a GN pinbox to avoid that particular pitfall. In fact, TECHNCIALLY, CO and many states specify the 2nd trailer is watercraft only. Overall, few states enforce any restrictions on RV's due to often vague statutes, or at least don't bother anyone unless they're real sketch.