Try
Womble
Just put both files on the timeline and export them as a single combined file
Ulead VideoStudio 9 or 10 would be another to try
Here
Both have downloadable trail versions from the linked sites
Infact all NLE ( Non linear editing) video software capable of accepting mpeg file as input should be able to do what you want. I only gave the above examples because i know they dont not reencode mpeg files unless needed( so called "smart encoding" ) thus leaving PQ relatively unaffected