Duplexing is where you adhere two different papers together and this can add beautiful effects to your work and example would be to print in colour with text and image to one sheet then foil onto a second sheet with a different colour and texture then stick the two sheets together.
Duplexing allows a varying edge colour and the ability to add extra effect by diecutting one of the sheets revealing the colour of the second sheet through the cut.
Triplexing is similar to duplexing but can add extra thickness to the finished product as it used three sheets. you could go on and keep adding sheets to for a product of say 5mm thick