S/PDIF is a stereo interleaved format, so I assume the mono units are just not using one channel. The only way to combine them is to use a digital mixer, or digital recording/editing program on a computer. I've seen entry level Tascam & Fostex digital mixers going for less than $500. On the computer front, you could bring the mono tracks into any sound card with a S/PDIF input one at a time, line them up and pan them in your software, then output stereo through the S/PDIF out.
Scott