For clustering Biztalk databases, go here: http://msdn.microsoft.com/en-us/library/ee308825(BTS.10).aspx
For multiple MessageBox databases, go here: http://msdn.microsoft.com/en-us/library/aa561513(BTS.20).aspx
You do not need a load balanced device to share the load, most BizTalk databases only allow a single running instance, so clustering is active/passive.
The MessageBox allows multiple databases, but load distribution is managed by BizTalk.
The DTA database has a single active instance, although you can create archive databases and use the Purge and Archive SQL Job to move data from the active instance to an archive instance.