Matching product #s on most newer RAM will let you run them in dual channel... even if they're not a "kit" or a dual channel specific memory (Ex: I have an old system with 2x 512 Kingston KVR DDR-400 Ram that were bought seperately and they work just fine together in dual channel).
The second set of RAM doesn't even have to be the same type as the first, as long as the RAM is in the correct slots. I have 2GB of Team Group (4-4-4-10) and 2GB of Mushkin (4-5-4-11) and they work perfectly fine in dual channel, I just had to set up the timings manually (which I would do anyways since I run tighter timings).
Project: Black and White i7 920 D0 | 3 x 2GB DDR3 | EVGA X58 SLI LE
XFX 4890 | Corsair HX750 | Corsair Obsidian