Put some ice into a shaker, add tequila and vodka. Shake, strain into a glass. 3/4 fill glass with ice and top up with orange juice. Gently pour the grenadine over the back of a spoon (tip of spoon touching orange juice) so it sinks to the bottom.
For garnish, make a slit in the orange slice, skewer the red cherry to the slice and place on the rim of the glass. Add a straw and serve immediately, without stirring!