Daughter and I played coop campaign for a couple of hours last evening. Was great. We needed to talk via XBL party chat, because game chat wouldn't, but it's really no big deal. There was no lag or weirdness that I've experienced before with other online Halo campaigns.
We discovered that the "default" controller setup is the same as that used in Destiny - scope zoom, grenades, almost everything. That makes it a lot easier to switch between games.
The party leader gets to choose which of the 4 characters in either team Osiris or Blue Team they want to play. Daughter chose the Masterchief each time. I ran as either Fred or Linda (for the sniper rifle). I given Locke when with Osiris.
We'll likely be playing MM once we've had enough campaign. Soon....