The first step is to convert the two samples into a single stereo .WAV or .AIFF file. Probably the simplest method would be to download MED Soundstudio 1.03c, start a new song file, load the left side into instrument 1 and the right side into instrument 2, and set the song to two channels in mixing mode and set the playback mode to 16-bit stereo disk save. Once you have that done, set the tempo to the slowest setting and extend the song length out to the length of the samples. With that in place, set the playback mode to 16-bit disk-save mode and play the song. It will prompt you for a filename, then you can save it as an .AIFF file.
Once that is done you can use the LAME encoder or some similar conversion utility to convert the .AIFF file you just created into an .MP3 file.
