Two dryers connected to the same exhaust via a "Y". Backdraft problems?
@Peter: I think you answered your own question in your comment to ChrisF.  What you want is an "inline draft blocker."  And you'll want two, one for each dryer.  Here are a few on Amazon:

http://www.amazon.com/Lambro-Industries-Blocker-1775L-Accessories/dp/B000H5PTJA/ref=sr_1_1?ie=UTF8&amp;s=hi&amp;qid=1304804320&amp;sr=1-1

http://www.amazon.com/DEFLECTO-BD04-Draft-Blocker/dp/B00009W3HB/ref=pd_cp_hi_2

http://www.amazon.com/Dundas-Jafine-DB4ZW-Dryer-Blocker/dp/B000DZDPPE/ref=sr_1_34?s=hi&amp;ie=UTF8&amp;qid=1304804226&amp;sr=1-34