Two things to say about that.
1) You can try to slightly bump up the ram voltage in the motherboard bios by .1v to see if it is stable with both ram modules in.
2) If that doesn't help, I would contact the ram manufacturer to show them this thread while asking for RMA of the modules.
Actually, where you may find the best resource for this type of problem is on the ram manufacturer's forum. If you use CPU-Z to find out your exact ram modules in use, tell them of it along with your motherboard model, they will have really great suggestions for bios settings I'm sure.
The manufacturer's forums are a truly excellent resource in this regard. Most have highly knowledgeable users in this regard, many actually working for the company itself.
In any case, please keep us updated on things. Interested in the final outcome and what eventually will fix this.