Well, if you took more than two powers, you haven't followed the rule that it must take two powers.
If you take 3, then you haven't taken 2. If you take 1, then you haven't taken 2. You must take 2, to take 2.
I do not have the Necron codex, so I can't comment on whether or not you can take more than two powers, but your logic is flawed here. If I have three apples, I ALSO have two and one apples. If I take three powers, I HAVE taken two, I've just taken one more. Math logic is pretty straight forward about this. Three is not a separate entity from two, it is just a representation of two with an additional one, while two is a representation for multiple "ones," or single units. Yay math!
Otherwise, if the entry says "Must take two" instead of "must take
at least two," then it is a no more, no less than two deal.
-Travis