If I am understanding you correctly, only one physical interface will be used for all traffic, correct? This is the default configuration, and how any MWG works when it comes out of the box. No special configuration is needed at all, just plug it in, run the initial configuration, and you will be ready to go (and write your policies from there of course).
Basically I'm trying to use a 2-server high availability setup while using a single physical interface for each server - I don't expect traffic volumes in exceess of 200mbps and do not need physical link resiliency.
Would this work by default?
Yes, this will work. The GUI is not sent through the HA proxy, but there is no reason that the single link won't work for what you want to do. I use the same setup in my lab in VMWare.