Pretty sure each set of chapter tactics is treated as a seperate codex.
Not quite, but you're still right. It's a bit weird though, I can't seem to find anything about not being allowed to take the same faction in multiple Combined Arms Detachments, though I'm not ruling it out just yet. Under
Selecting Detachments on pg. 120 it says again that 'you can include any number and type of Detachments in a Battle-Forged army provided you have sufficient units' with no reference to Factions at all. However, it's not that which puts the kibosh on the SM allied fun, the SM FAQ states in replacement of the Allies section of their Codex:
Space Marines can be taken in an Allied Detachment even if your army's Primary Detachment contains units with the Space Marines Faction, provided they have a different set of Chapter Tactics. Units in these Detachments are treated as Battle Brothers
Which I think is what proves your point irrevocably. I guess I'm spoiled though playing IF, BA, and DA. They're all separate codices, so I'd still be free to ally in a White Scars Allied Detachment no worries as I would only have the two allowed Detachments as per the FAQ.
Nuts! Sorry to sidetrack the thread with such thick text. At least it kinda pertains to the subject...:blush: