Duplexing is essentially gluing the front and back of the card together. There are a few different reasons why we’d do this.

First and most obvious is to create a custom paper combination. Most all papers are the same color on the front and back, so duplexing can create a very unique paper for your project.
Certain stocks don’t like to be duplexed to other stocks, but we have experience with this and can make a recommendation to suit your needs.

Duplexing is also a great way to have a two sided card without any of the impression showing through the other side. Since letterpress is a pressure process it’s inevitable for the print on the front of a sheet to affect the backside which can minimize this with a more conservative impression. With dplexing you’re left with a double thick card with a beautiful clean impression on both sided.