Hi abbeyman,
It depends if your printer supports it or not. From a PrintShop Mail point of view, this is done by applying a different Page Setup to each layouts, where some layouts will trigger duplexing, and others will set it back to simplex. Apply the Page Setups to "Current Layout" to apply to the current layout that you have selected.
However, this will only work if your printer supports it. Many printers do not support mixplex, so even if you configure your PSM document properly, it won't allow you to do something that your printer cannot physically do.
Regards,
Raphaël Lalonde Lefebvre