two mail accounts one exchange 2000 svr

I am putting a 2000 exchange server into a small buisness the problem I am having is that the people have 2 email accounts as they run 2 buisnesses they want to recieve and send from both accounts, ie b.smith@network1.com and b.smith@myaddress.co.uk, all with out having to close outlook and change profile the exchange server is set to recieve the e-mail from both of the smtp accounts but how do you tell the server and outlook to give the users the choice of sending from b.smith@network1.com or b.smith@myaddress.co.uk.