Do you mean send the same signal to both at the same time? If you're running off the soundcard, my guess is that you have a cable with a 1/8" jack going to RCA outputs. You could get another of those and one of those plugs at Radio Shack that goes from one to two 1/8" jacks. A bit of a kludge, but it will work.
You could also build a box that will do the same - ask in the DIY Forum.
Uh...sorry can you explain it in a more simpler way? I still don't get it. My companion 2 uses the aux port and the companion 5 is usb port but I don't get how you get both to work at the same time with my desktop. Please tell me how to do it in a easier way. Thank you!
I don't think it would be a good idea at all to try to combine them. They both use this technology:
Proprietary TrueSpace[size=0.8em]®[/size] stereo digital processing circuitrycreates a wide two-channel soundstage engineered specifically for listening at your computer.
So basically a sound-enhancing digital magic voodoo thingy. Now when you have 4 speakers creating 2 different "true spaces" it might or more likely might NOT work very well. These systems are designed NOT to be extended with additional speakers.