Sorry, I know this thread is a bit old now, but I've owned a 4.2
I also bought 2 2.1s separately and put them together into a 4.2. I actually preferred the separate volume controls since you can control the rear speakers completely independently of the front instead of setting rear volume as a fraction of total volume. I'd recommend taking the control pod off the rear speakers so you can mount it up front with the other pod. I actually removed both pods and mounted them under the desk, then wall-mounted the speakers.
And the setup sounds really good. I had a 4.2 with an SWS sub added on (Klipsch PLEASE bring these back!!!) and it sounded amazing. I made the mistake of stacking the subs though, and one of the 2.1 subs fell and ripped the connections off the back of the other
So don't stack them, I went from a 4.3 to 2.2.
A good upgrade down the road is to replace the front speakers with a good set of bookcase speakers, the 2.1 sub puts out enough power to power a decent pair. It makes a great sounding setup and keeps things clean since you don't need the receiver, amp, etc... It's too bad the SWS sub was discontinued because the 2.1/4.2 +Bookshelf speakers+SWS makes a more than respectable home theater setup for a good sized living room.