They are not the same thing, actually. "To date" is similar to "at the moment" - which implies that something is known to be true right now, like a fact; it is like waiting for something to discredit it or make it no longer the known fact. EX: The school teacher has taught in 2 public schools and 1 private school, to date. (Yes, "to date" can be used ANYWHERE in a sentence - as an adverbial.)
However, "so far" is used for on-going actions, thoughts, ideas; think of it like saying that you know something now but expect a change soon. EX: We're baking cookies for the show tonight and, so far, we have 12 cookies done; in 2 more minutes, we'll have 24.
Hope this helps!