Here is a Navien Q&A guide to common questions

Q: Can I twin Navien Furnaces?

A: Yes you can twin Navien furnaces. Since this application typically has both furnaces sharing a return, you either need to have backdraft dampers to prevent the other blower running backwards, or if you don't have that option to do that, make sure you run constant fan and set each furnace for 450 cfm minimum to prevent this from happening.