2 things, after i went back and looked at your first post again I've come up w/ 2 possibilities.
1. Was the CPU oc'd when you put in the second stick of ram? Because from what I understand when you oc it will usually automatically change RAM values so you might have changed the values for one stick, but not the other so when you put them on seperately it reads them as individuals, one cranked up, one stock, which would cause a problem with dual channel because its reccomended that they are the same ram chip. And if one is messed with then they are different. If this might be the case, return all the values to original and put both ram chips in. there should be a button to return all bios settings to original values.
2. When you returned the other ram chip you might have gotten back a different ram chip then the one you purchased. So if you can, free of charge of course or with minimal cost to yourself, return both and get a new set of 2 from the package it might be worth trying that.
those are my 2 hypothesis' maybe one will work out.