One of my characters has both Mythic Hellfire Citadel skips completed from a while ago. On that character, I can skip+skip right up to Mannaroth.
On any given alt, I can zone into M-HC and take the first skip portal, but at the point of the next portal, Khadgar’s just standing there, offering me the second skip quest instead of the portal already being open.
The second skip quest text does mention “your warband previously did this” but the toon actually standing there isn’t benefiting from that fact.