It's funny ! i had this feeling about the "hen-egg" problem...
So, to be sure i understood : if i have a domain aaa.org and a second domain bbb.org,
can i use for aaa.org :
primary DNS : bbb.org
secondary DNS : aaa.org
and for bbb.org :
primary DNS :aaa.org
secondary DNS : bbb.org
I am sorry, i only start to put my nose in those "concepts" but feel it still the same aviary problem !