Test one module at a time to see if it can be stable, or if one performs differently than the other.
If both are completely stable, then some tweaking may be able to stabilize it.
Let us know the result.
Also, initial slots for a motherboard are slot 2 and 4 away from CPU.
http://asrock.pc.cdn.bitgravity.com/Manual/QIG/Fatal1ty Z97X Killer_multiQIG.pdfWhat is the reason you want to use slots 2 & 4?