For information about domains and their relationship to server
instances, see
http://docs.oracle.com/cd/E18930_01/html/821-2416/ggoek.html#scrolltoc
If you use one domain with a test and production instance, then you can
use one admin console to manage both instances (i.e., you only need to
login once). You could even share a named configuration between the two
servers so that the configuration stays in sync. This all requires using
a single domain.
However, if you want more separation and independence between the
servers, such as being able to upgrade the GlassFish software on the
test instance before you upgrade the software on the production
instance, then it would be appropriate to use two domains.
Tom