I know both Recover and Roost heal 50% of the user’s maximum HP, while Roost making the user loose the flying type temporarily. But they each only restore 50% and they don’t stack on themselves. So does Recover and Roost Stack on each other? ( For example a Smeargle used roost, then the opponent’s move doesn’t deal damage and/or misses, and Smeargle uses recover ).
PS: I’m not using this strategy for Smeargle by the way. It’s just an example..
It's not a situation where they would or would not stack.
Functionally, the 2 moves do almost the same thing. Recover calculates 50% of the user's max HP, and restores that much HP to the user. Roost does the same thing, while also removing the user's Flying type if it has it, and overriding effects such as Levitate and Magnet Rise until the user moves again.
The only reason I can think of to put both moves on the same Pokemon is if you're worried about having enough PP to continue healing for a long time. But that doesn't strike me as a particularly valuable use, since you're also giving up 2 move slots on the Pokemon in question, critically limiting you options in battle.