MS Exchange, 2nd domain

Ok, here's the deal:

We currently have 1 server (Windows 2003 server sbs).  On it is MS Exchange 6.5.  We have (a dynamic IP) cable internet access through a netgear router.  DNS2GO

We have a client that wants us to host their email on a server located in our office.  So, we are currently loading Windows 2003 server and MS Exchange on another HP server.  We will probably have DNS2GO loaded and configured on that server as well.

I have a few questions:

1) Will it be easier (or even POSSIBLE) for the Exchange that we have on our main server, to manage another Domain?  Or should we just continue loading the 2nd server with Exchange and manage it separately, within our network?

2) Is it possible to have 2 Exchange servers (not connected in any way, to each other), installed under the same network (2 different domains)?  If so, how is that going to work (as far as WAN IP's....email routing, etc...)

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if you guys think that we're attacking this the wrong way, please let me know.  How exactly would YOU handle a situation like this?

thanks for the info