It would only happen if this happened:
The dual-type move has a 50% chance of being one type or the other.
For example, if they made Solar Beam a Fire/Grass move, half of the time it would end up as a Grass move, and the other half a Fire move. See what I mean?
Ceci n'est pas une signature
I walk through the valley of the shadow of death
Shippings I support: Ash/Misty, Drew/May, Kenny/Dawn, Ash's Pikachu/Dawn's Buneary, Tracey/Daisy, Cilan/Iris, Clemont/Serena, Harley/Zoey, James/Jessie, Max/Bonnie, Max/Mira, Max/Vivi Winstrate (I multiship Max and only Max), Butch/Cassidy, Gary/Lillie, Kiawe/Lana, Sophocles/Mallow, Lunick/Solana, Wes/Rui, Brendan/Game May
RIP Blake Carrington, Ben Matlock, Joe Mannix, JR Ewing, Jim Phelps