Ah ha, that is an entirely different situation then
user1@domain.com not being able to see
user2@domain.com in the GAL.
In fact to get c1 and r1 to see each other in the GAL you would have already needed to modify the GAL configuration since by default you can only see results from within your own domain.
For a good overview on some of the many different ways you can possibly work around this problem take a look at this post:
http://www.zimbra.com/forums/adminis...tml#post109525