At this time there are no plans on incorporating sharing between multiple domains on the same server. We get this request a couple times a month in both our sales and support departments so it should already be on our radar for future considerations.
I'll be glad to submit a feature request on my end and will send this directly to the developers for further consideration.
In my writeup to them I will include the feature request for linking shared resources (Contacts, Calendars, Chat, etc) on multiple SmarterMail domains within the same server, as well as public links for shared resources.
Technical Support Supervisor