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Hi,
Are the PRD and DR site on different physical locations? If no, then it's maybe better to use a sql cluster together with a biztalk farm. You will have the same number of machines but when sql A goes down (not the site), there is a lot less down time using a cluster.
If you have different sites for PRD and DR, then you could use BizTalk log shipping. Using BizTalk log shipping, the database backups will be copied towards the DR site. If the PRD site goes down, then you have execute some steps in order to fire up the DR site.
There is a complete documentation from Microsoft how you can configure BizTalk log shipping and which steps you must follow in order to fire up the DR site.
Regards