I had to have the same thing working so on our setup I had Zimbra setup on a different domain (i.e. main company e-mail is company.com, zimbra domain is companyltd.com). Everything comes through a separate gateway server which filters everything then forwards to the specified server.
So in the case of company.com I have a .forward file for each person's account which will receive e-mail for
person@company.com but at the same time forward to
person@companyltd.com
Obviously this setup assumes you're running postfix on your sendmail server. If there's another way to achieve the same result I don't know.
I know in Zimbra you can set up forwards for each account that the user can't see but again I'm using this with 2 domains.
If you want all e-mail to be delivered to 2 different servers at the same time for the same domain, I'm not sure if that can be done, I know I looked into that option when I began testing and couldn't find a way to do it.