Board Thread:Questions and Answers/@comment-74.132.212.254-20130609021048/@comment-1757231-20130718032221

Simple Answer: It's a Net Boost. In other words, it would be added on vs. multiplicative. Watches don't speed up other Watches due to in-game problems if they did.

Complicated Answer: It's still a Net Boost. Watches on the Pedestal in source code don't count it as a machine: therefore it cannot be speeded up by other watches. Watches in player's hands do speed up other machines, but are not sped up by Pedestal Watches or vice versa.