You actually need to know the store's policy, in the event it says they will accept both, and a checker refuses. This happened to me at a store I frequent a few days ago, so I asked to speak to a manager. When she came I told her what I wanted to do and she started to say I couldn't use them both so I pointed out their policy to her, she read and re-read it and instructed the checker to do the transaction with both.
This has happened several times with me at various stores, individuals try to make up their own policies or don't know them, and I have to say, policies change so YOU have to stay current on them as it's not the store's job to save us money. Know the ones that will accept both and those that don't.
It's fun to save $$$!