If either Cluster Missile attack fails, do I get to avoid discarding it? Or do they both need to miss?

They both need to miss to trigger munitions fail safe.

Also, for future reference - you can use the search bar at the top of the page to find posts that will have answers to a lot of questions you may have (a lot of people don't know it's there). For example searching for "cluster failsafe" would bring up several threads about this topic. Hope that helps with any other questions you may have!

I'll keep that in mind. Thanks.