The newest version (USA version 6.1) has a bug in it that prevents the duct page of combined systems from displaying properly. It usually looks like the below.
This occurs when clicking on the Enter Multiple System Details and it only displays half of the grey popup as seen below.
The current workaround is to use a prior version of the software in order to input the values properly. If you don't have an active prior version of the software you can download it from this link:
http://www.energygauge.com/downloads/setups/usa6002.exe
Icon of the prior version: