You need to create a BizTalk Server Group with two servers in that group. This will provide both high availability and scalability for message processing. The message box should be available via a clustered instance of SQL Server. You need two additional servers with Microsoft Windows Clustering Service installed in an active-passive configuration. This way, if one of your BizTalk nodes goes offline the other will pick up the work automatically. If both BizTalk nodes are up, they share the work load. Lastly, if either of the SQL Server nodes go offline, the other will pick up the work load. However the SQL Server nodes cannot share the work load since they are in an active-passive configuration.
Ben McFarlin
BizTalk Consultant