My raid is a full clear on normal this week, so I got a guildie to make a raid with me, and zoned into Dragon Soul on heroic to see if Morchok would be up.
He was not up, and the entire raid was cleared. Even after cycling between normal and heroic, it was still empty.
While the first answer is correct as well it's not the case for the second link though. We've cleared the first bosses on heroic first, then switched to mains who were not in for the heroic kill + alts to kill the same boss on normal right after. Simply a matter of no loot getting wasted. They just have to be killed on normal the second time so that it's possible to switch all mains in for later heroics without ID issues.
You will notice there won't be the same players for the heroic/normal kills.