Avarice has hit the nail on the head. Best Answer material for sure.
Yes, it will function. As long as they are both standard DDR3 (or 2 if that's what you have) they should function at the settings of whichever one has the loosest (slowest) defaults. However, if you want to get the most extreme possible performance, there are reasons that memory is sold in kits of 4 instead of picking up 4 individual pieces. One of those reasons is the advertising power, of course. But running 4 modules in dual channel config calls for some slack and some modules are designed to run that way.
There was an article recently - I'll look for it. EDIT: http://www.tomshardware.com/reviews/32-gb-ddr3-ram,3790.html