First there is no way to "Alternate" delivery in exchange. It is First come first serve when you are using Internet E-mail instead of traditional exhange mailboxes. There is really no way around it.
And you are correct with Distribution lists, they will both get a copy no matter what.
However you do have 2 solutions, The first would to get them both on outlook instead of express. And configure it for corporate or workgroup use so that you can use exchange.
Create three mailboxes 2 for the sales people (box a and b)and one (lets call it box c) for the e-mail address.
Send all e-mail to this single mailbox (C)and give them both ownership and send as rights to the mailbox (C).
Each of them can then open up that mailbox
(tools/services/exchange/properties/advanced/add mailbox)
and see what mail has been delivered to it.
This does not affect delivery to the box's a nd b since it is being delivered to box c.
Solution 2
Set up the single account on their machines asan IMAP account. This will mean that you will need to enable IMAP at the exchange server. This will act almost like an traditional exchange mailbox. The downside is if both people are in it at the same time they will not be able to see what the other is doing until the next update period.
My honest opinion is get all of your users to use Outlook with exchange. You will find that your e-mail admin woes such as these will disappear. If your company has spent the money and time deploying exchange server, you might as well exploit it to it's fullest potential.
I did and I swear by it. Mail admin has never been easier.
