No you can't buy different sets, if , you mean one set dual channel and one set tri or quad channel.
I mean 2 different sets of (2x4) with the same frequency. But I'm wondering this because I have an msi p67a gd80 that has dual channel memory but I got a 12gb vengeance set as a present and that's triple channel. That won't work will it? ...since its triple channel. I would have to buy a new set that was dual channel for it to work wouldn't I?