Well, here's my typical canned response to those types of issues, and while it IS sort of generic, it really is true.
Anytime you plan to use more than one module, for dual, triple or quad channel operation, is is ALWAYS a good idea to buy them in a matched, tested set that is already known to work together. Even buying IDENTICAL part numbers is no guarantee that any two modules will work together because there are so many variables involved. Usually there is some room for compatibility differences, but a LOT of the time, more and more as time goes on and systems become more specifically finicky to differences in hardware, there is not.
Modules with the part number XXXX this week might use different memory chips, from a different chip supplier, with different numbers of chips or not even having the same number of sides containing chips, then the same XXXX part number that came from a different batch of chips next month. That means they may or may not work together in multiple channel configurations, or at all.
In some cases, even modules that came off the exact same production run but were not tested to be compatible with each other before they were packaged up and sent out, ie, bought separately NOT in the same package together, might not be compatible. I've seen this MANY times where somebody buys two packages of the same module, rather than one package with two modules in it, and they would not work together although each module worked fine individually. It is NOT uncommon. It is also NOT in every case.
Sometimes you can slap two totally different modules in together that are not even the same speed, voltage or latency and they will simply work. Often though this is not the case. It is ALWAYS a crap shoot unless you buy them together and even THEN sometimes it still happens, but a LOT less frequently than when bought separately. For examples of this you can check out this thread here where I provide specific examples of how modules that are even the same model and part number, are hugely different.
http://www.tomshardware.com/answers/id-3610013/amd-ram-compatibility.html#20562100
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