The answer, by the way, is "no one knows, and GW didn't even think about it" which is part of how you know this codex is poorly thought out and shouldn't be allowed.
If I had to guess, I'd say you are still flat out barred from bring "come the apocalypse" allies, in any combination, in the same army, regardless. This is just a guess, though.
The real question is can you join, for instance, Tau and Inq, who are desperate allies, to the same SM unit, for which they are both Battle Bros? Would the unit fail to do anything on a 1/6 then?
Who Knows?