Recently I have seen multiple MdC on short haul flights, as many as 3 out of the 4 total crew. I was told they decided who was 'doing the role' of MdC or number 1 as some say based on seniority factoring in whether that's primarily on short haul or long haul (last week one had been MdC longer but mainly worked long haul so was the number 2). Sometimes both MdCs work in business, sometimes one at the from and one at the back so clearly the protocol isn't too prescriptive.

I've also seen the same crew on night stop switch positions overnight in terms of who works with the MdC in business so it isn't set in stone or a particular process by any means.