If the modules happen to work together, then you will get dual channel. However the problem you might run into is compatibility between the two - e.g., one module being one manufacturer @ 1333 and a different manufacturer @ 1600.
Ideally you want to match exactly to insure dual channel compatibility.
RAM is quite cheap these days, it wouldn't be too much more expensive to just buy a kit of 2x4GB at 1600. This way you can insure they'll work together. Plus, if you're thinking of OCing, doing the kit route opens the door to start doing that now.
FWIW, make sure you're using a 64bit OS; 32bit is maxed at 4GB.