I dont buy into the idea of a second position particularly. You can have flexibility in players for sure, so somebody like Huggins can play left or right. Rigg can probably play left side or right side 8. Huggins is a full back though, not a CB. Generally though the role of the utility player, as it once was, isnt the way forward anymore. I see the likes of Milner as the end of that sort of squad role.
Coaches, generally have a little amount of time with players once pre season is done. Expecting RLB to be able to coach Seelt to be a top class CB and a good forward thinking right back is unreasonable. There is no time. The role of the CB is very very specific in modern football. A huge amount of technical detail goes into how to play it. A defensively solid full back like Hume could slot into a back 3. A defensively sound CB doesnt translate well to a forward thinking full back. At our level anyway. Elite elite players like Maldini or Lahm can make it look easy, but that isnt down to coaching, that is down to huge talent and experience.
The further up the pitch you go flexibility is easier to create, and trust. Trust being key. A player like Mundle, for example could be effective in a couple of positions. Indeed you could play a fluid front 3 if the players are talented enough. But the reality is defence is much more structured and organised, attack can be more fluid and free form.