I have 2 wmv files i am trying to join using asftools, and it comes up with an error, when going to the second file (when joining)

I have looked at the 2 files there are differences in the propeties

Audio: diferences in bitrate and also in channels (one stereo and one mono), and audio sample rate.

Video: diferent data rate between the 2

What do I do in this situation to get the files joined?? Do I have to convert the better quality one (stereo) down to the mono type file??