Recommendations for managing two companies on same Domain

The companies do work quite close already, and yes I agree it is not the best practice in theory.

Unless there are show stopping circumstances, I will continue to manage both businesses (who physically are in the same premises) with the one SBS 2008.

I have already started separating them with Security Groups and Email Policies, but since I am new to SBS 2008 I wanted to see if I should also look at creating new roles for each etc.

When you say setup two domains in exchange, do you mean to setup two authoritative domains?

If you can afford it, create a parent domain and two child domains. (Corp.com, company1.corp.com and company2.corp.com) Then formulate trust.  http://support.microsoft.com/kb/255248

ieden, that is impossible - SBS does not support sub/child domains.

Probably at this stage, since we have already deployed the server to the client, they would not be interested in investing additional money in my time to set this up.

But I think this is good advice.

ANy other ideas then?

http://sbs.seandaniel.com/2008/10/hosting-multiple-domains-on-sbs.html

BTW further info here:
http://sbs.seandaniel.com/2008/10/hosting-multiple-domains-on-sbs.html