I just ran into this sentence in an online article:
But as the App Store’s fortunes rose, so too did the iPhone’s, and later the iPad’s.
If I were editing that sentence, I would remove the too on the theory that it doesn't add anything to the sentence's meaning beyond what so provides. On the other hand, the phrase "so too" (or "so, too,") is quite common in everyday speech and writing, and it may be idiomatic for some speakers and writers. Site searches for "so too" and "so, too," reveal that these terms appear a total of 25 times on EL&U pages—mostly in answers, and mostly by extremely well-informed answerers.
If we think of so in the quoted example as meaning "in a like manner" and too as meaning "in addition," it seems to me that the sense of too is embedded in the sense of so, since Event B can't happen in a like manner to Event A unless it is happening in addition to Event A. But perhaps I'm overthinking this and the main point of too is simply to emphasize the also-ness of Event B.
Here's my question: Is the too after so in sentences like the one quoted above superfluous to the sense of the sentence, or does it contribute a shade of meaning that I may not be recognizing?