Converting two mono wave files to a stereo file or mp3?

Greetings folks,

I've been recording a local Friday evening concert series on my A&H Qu-16, which is turning out to be a fantastic little mixer by the way, but am puzzled how best to deal with the resulting wave files. The Qu series record every armed track to a separate wave file, including the Main mix which is recorded as two files, one left, the other right.

Does anyone know a convenient way to convert these two mono wave files into a stereo version? A few of the bands have the tools to mix down the individual tracks, but most are just interested in the Main mix which is not particularly useful as two mono files.

Cheers,