Two exchange servers in one domain

Hi,

Have the following situation:

One Windows 2000 domain/AD installed, two sites: company A and company B have a DSL/VPN connection. Every site has its own Exchange 2000 server and internet domain:

site A: a.com
c.com

site B: b.com

Some users at site B should have also an c.com e-mail address.

I have a few problems/questions:

1. users at site B can’t send e-mail to a.com (or c.com) e-mail address (and vice versa).
2. Is it possible to replicate all mailboxes? (sothat in case of a site A server crash people can access their mailbox on Exchange 2000 Server located at site B.
3. How to setup routing groups/organizations/etc.