I've actually run games that fall somewhat into this category that Cai brought up in the original post here, but not exactly. My first game - and surely my longest, as it began on another site in 2010 and transferred over to MW in 2012 before continuing on another few years - began with PCs at level 2 (Pathfinder 1e, sandbox style story). Once those PCs hit level 8 and transitioned to a new plane of existence (long story), a new set of PCs were created and were run in alternating chapters with the original PCs; players would switch from mid-to-high level characters to low-level characters and back again every couple months or so. This worked pretty darn well, honestly. It allowed players to bask in the glory of upper-level antics that often grind games to a halt and then suddenly they're fighting for their lives again against kobolds and goblins with their low-level new characters.
I won't pretend this approach will work for everyone - I was too new to know better and my players were too damn kind to tell me I was nuts for trying it - but it was a fun twist on the two-characters/one-player concept Cai alluded to above.