Another Ken Silverman utility. Ken has this to say about ZipMix:

Sometimes, when you run 2 different .ZIP programs on the same data (PKZIP & KZIP for example), some files get compressed better in one program, while other files get compressed better in the other. ZIPMIX will take the best files from each one and create a new output file, which is guaranteed to be equal to or smaller in size to both of the input files.

http://www.advsys.net/ken/utils.htm#zipmix