In general, you can learn multiple upper tier headers, especially if you are learning things that are highly related to each other (diversifying is harder than specializing, but not impossible).
Also naturally, learning multiple things simultaneously is slower than focusing.
However, there are some upper tier headers that "lock you out" of some other options - we promise, whenever you look into one of these headers, you will be told very quickly what you are giving up. Druid happens to be one of those upper tier headers that imposes some restrictions - another example might be Cornelian Knight and Necromancer.