So, basically, the verbs in each case require the same object to be in 2 different cases, which is impossible, so 2 pronouns are used to overcome this problem. That also explains why the pronouns are paired, the "то"-side is a demonstrative and the other side must be its interrogative counterpart. Which means the pairs are:

то, что (what)
то, кто (who)
такой, какой (what kind of)
так, как (how)
там, где (where)
туда, куда (to where)
оттуда, откуда (from where)
тогда, когда (when)
столько, сколько (how many)
потому, почему (why)

http://www.alphadictionary.com/rusgr...n.html#tkpairs

Thanks, guys, you've helped me a lot. I've also seen "То, что" at the beginning of a sentence, but I'm unclear as to what it means here. Having "То" as a whole clause seems odd, unless it's as simple as a clause swap. "Мы говорили о том, что вы читали" -> "о том, что вы читали, мы говорили" (I'm not sure if this is right, call it an educated guess :P).