2 ram sticks not working toghether
I would start with blaming the RAM. If it is possible to return both of them and get a replacement, I would do that.

The only way to tell if it is the RAM or the slot is to try different RAM. The problem with that is if you show up at an Apple store with 3rd party RAM installed saying that you have a bad slot, they will take out the 3rd party RAM and say "problem solved."

One option is to return the RAM for a refund and buy new RAM from another vendor, but if it fails you'll likely get a similar reaction from an Apple Genius.

It's a bit of a Catch-22 unless you can get an Apple genius to test your Mac with some Apple branded memory. Not sure if that is possible or not.