I tried to print a duplex postcard today and merge the mail list in ID CC and it REFUSED to give me the option to do multiple up on a sheet! It will do it no prob siimplex! I swear I done this before in earlier versions of ID!
Mulitple records per page only works with single page templates, and that's the way it's always been. It's better to do this sort of job as two files, one for the merged data, and a single page for the static that you print N copies from.