In RAM you can have a couple situations.
1. A ram module itself so 1 module could be bad. To figure this out you need to test each.
2. Combining 2 modules may be bad. They may be a mis match thus causing errors. Mostly caused by timings on two different types of ram.
3. You can have ram that is totally fine by itself but with another stick they don't like each other. Sounds crazy but it is true and very much possible.
I would suggest testing each stick individually.