This can only be done via Windows 2000 Advanced Clustering and Exchange 2000 Clustering. In this scenario you have two separate 'boxes' that share a common data storage.
This proivdes load balancing across two separate 'boxes (A&B)' and also fault tolerance - if 'box A' goes offline all users on 'box A' are automatically transferred over to 'box B'.
The key here is that you are sharing a common storage solution where the database(s) is residing. Such as a Storage Array Network.
This provides hardware redundancy against the server boxes and software redundancy aginst the OS and Exchange application. It does not provide redundancy against the databases (priv.edb -.stm and pub.edb -.stm). To accomindate for this you will need to ensure that the storage array in use has built in redundancy for power supplies, controller cards, drives, write back cache, etc.
On the 3rd party side Legato carries a product that is typically used with Co-Location. It allows you to mirror one server to another location. It does work with Windows Clustering and will provide an extra level redundancy for the private and public databases (web store).
