Not for a symmetrical room. In fact if you read Todd Welte and Floyd Toole's paper on subwoofers, you will find that placing two directly opposed to each other has a very interesting effect on standing waves. Where the peaks would normally occur the two are working against each other and where nulls would occur, they actually are working together and bringing the nulls up. It gives a large area a flatter response. They went on to show that 4 subwoofers can optimize this. The simulation went from 1 to something like 5000 subwoofers.
Here's a link to the article:
[url=http://www.harman.com/wp/pdf/multsubs.pdf]Multiple Subs[/url]
Here's a link to the article:
[url=http://www.harman.com/wp/pdf/multsubs.pdf]Multiple Subs[/url]