exchange child domain

I have a domain an a child domain, in the main domain there is an exchange server, in the child domain there is also an exchange server.

1. I want the users in the first domain to be able to access the exchange in the child domain
How can i make posible for the user of the main domain (mydomain.com) to be able to login in the child domain (child.mydomain.com), using mydomain.com user name and password?

2. My intention is also, shout down the main exchange to rebuild it but meanwhile have the other MTA (in the child domain exchange server) distributing the incoming messages, will that work? will the second exchange distribute the incoming emails while the others is being rebuilt.

thanks all