Suppose that I resolve Seasons Past and wish to return Turn // Burn with it. What other mana costs may be returned alongside it? Does it take up the 5 CMC slot, both 2 and 3, or something else entirely?

A split card has two converted mana costs. In the case of Turn//Burn, its converted mana cost is 2 and 3, not 5. If you wanted to return Turn//Burn with Season's Past, it would count as you card with a converted mana cost of 2 and your card with a converted mana cost of 3. You would not be able to return another card with a converted mana cost of 2 or 3, but you could return something with a converted mana cost of 5.

Nathan Long
Magic Rules NetRep