A space marine detachment chosen from this codex that has one set of Chapter Tactics may ally with another Space Marine detachment chosen from this codex that has a different set of Chapter Tactics. For the purposes of the Allies rules these detachments are treated as if they were chosen from two different codexes.
This is the first part of the rule, the 2nd being quoted earlier.
You can have Ultramarines Primary Detachment and Imperial Fists secondary.
As far as running 2 primary detachments from two different chapters, look at page 109-110 in the
BRB about Primary Detachments. If you play a larger game you can take an additional primary detachment.
Now go back to the rule I've quoted above and you can see that in no place does the word Primary or Secondary appear. It simply says detachments. You have permission to take a second primary detachment from the
BRB (in a large game) and no restriction from the C:
SM on that second primary from being another Chapter.
So what this means for C:
SM
- You are allowed to have a primary detachment and a secondary detachment from different chapters
- You are allowed to have two primary detachments from different chapters.
- You may use successor chapters interchangeably and it will count as one detachment
- You may use successor chapters mixed within 2 primary detachments
The codex has been written specifically to allow a wide range of Space Marines to appear on the battle field. You could have a Primary detachment of Ultramarines mixed in with Howling Griffons, Silver Skulls, Novamarines and any other combination, then in your second Primary detachment have a White Scars army with Storm Lords and Dark Hunters all mixed in.