I was going through my
brb tonight and had a thought about building an army based around the idea of soups. Not that I wanted to try and do so myself, but I have seen and heard people saying that you couldn't take a battle-forged army or something unless there was at least one thing similar in their faction keywords. After glancing through the
brb and a quick skim in my codex, I haven't seen anything that forbids me from taking a detachment of Tau and another of, for example, Eldar. I have seen multiple cases and points in both that have said each detachment must be from the same faction, but nothing saying detachment needed to be the same in a single army.
Also, I will apologize now if there is a simple answer and I'm simply blind to it. I don't have very errata,
faq, document open right now in front of me and I've been up for 18 hours from work and such. Please bare with me here