If Pokémon #1 uses Rain Dance but doesn't hold Damp Rock then I swap in Pokémon #2 and it hold Damp Rock will rain dance last longer, or does Pokémon #1 need to hold Damp Rock?
The Pokemon that uses Rain Dance must be the one to hold Damp Rock. Another Pokemon with the item will have no effect on the duration of rain.
Source: Experience and Damp Rock (Bulbapedia)
