Interesting Scenario ?? is there any solution availible

We have got to 2 domain of diffrent forests. Domain A and Domain B

Domain A is windows 2000
Domain B is windows 2003

Both domains has saperate Exchange 2000 and 2003 installed and working
Both domains has isa 2004 installed and working

Want to do:::--

1- want Domain B users could use Domain A proxy or isa 2004 server for
internet
2- want Exchange servers of both domain can send mail directly to each other
using internal LAN rather then internet.

How can i achive this... need your help

thanks

zr