I think it may depend on the brand and specifications but at one time I bought 4gb of gskill and then later upgraded with another 4gb gskill kit of the same kind. Pretty sure you'll be ok though.
I do know that full kits of the size you are looking for have a qc factor to make sure all the dimms are capable of running at the projected frequency and timings. If you get a different brand you can tweak in your bios to get the one kit running at it's timings while the other kit runs at a different setting given you have the motherboard. Most users don't like to go to that sort of trouble though.