I think This update is solely a Spy update, not a dual update, for the following reasons:
- When the Spy update bit was released, they taped "SPY" over the phrase "The Sniper Update", as thought the Sniper was being replaced by the Spy.
- The lameness of the Sniper equipment.
- If the update is coming out Thursday, they only have two days to release the remaining update information. It would be easy to release information about hats and the new system of getting items one day, and the alternate Spy Knife the other; adding in the Sniper's alternate melee weapon and Achievements would make for two very crowded days. (Not that I'd complain)
- If you look at the TF2 Blog, the link is to the Spy Update, not to any Dual update.