I have a couple of pretty powerful 2.1 speaker setups - at least they are more than powerful enough for my small living room. MY mobo has built in 5.1 sound card with x number of 1/8" jacks. Is there any way to rig up a harness to connect the 2 2.1 speakers to make it a 5.1? I know that i would have to add a center speaker in there somewhere.

Assuming that you're talking about normal 2.1 powered speakers for use with computers, the main issue is you'd only have stereo inputs. The easiest thing to do would be to disable the subwoofer channel on your PC, giving you 5.0 output which should route the sub channel to the front left/right. Then run the front 2 channels to one system, and the rear 2 channels to the other. You'd need some other powered system for the center channel, OR you could disable center and live with 4.0.

If, that is, your computer allows you to disable the center channel. Your center sounds would then come equally from the right and left speakers.

Meanwhile, if there's a jack for rear speakers, connecting the second speaker system to that jack should give you rear signals, unless, as Daniel fears, you really only get two channels out of the bother moard.

It's unfortunate that you wrote "x" jacks. If there's a third jack, that would be for center. One channel of yet another 2.1 system would add your center speaker. You would then have, making a comparison to jumbo jets, a mosquito-sized and mosquito-powered surround system.