This technique is usually used for HT when the subs are in mono or LFE. You can cancel certain room modes by placement with 2 subs much better than one. On our resource page there are a few articles that can be very helpful, possilbe the most helpful is one by Todd Welti and Floyd Toole of Harmon International on how many subs. For this technique to work the subs do have to be identical.
Rives Resource Page
Rives Resource Page