I use the following method with Cubase without the need of any dedicated M/S encoder/decoder -- I'm certain you could do the same with Audacity . . .
1) Take a SDC mic and a LDC condenser in a figure 8 configuration;
2) Point the SDC at the guitar (or whatever you're recording) and find a sweet spot;
3) Put the LDC directly under it, getting the 2 mics as close to each other as possible, with the left and right nodes of the figure 8 perpendicular to the sound source;
4) After recording, duplicate the LDC track and open the mixer;
5) Link the 2 LDC tracks, pan one hard right and the other hard left;
6) Put one of the LDC figure 8 tracks out of phase;
7) Mix the linked LDC tracks with the SDC track in a ratio you like -- as you increase the LDCs, the stereo effect will become more pronounced.
Here are a couple examples of something I recorded with this method. It's Youtube, so the quality is Youtube-limited but I think you can clearly hear the stereo effect. . .
This one has quite a bit of the LDCs mixed in:
http://youtu.be/VGOZgui1YRw
This one has relatively little:
http://youtu.be/wt0H3GAdQ7I