Medicare Advantage Plans

[tsrl01]

We have two retirees who are both enrolled in a Medicare Advantage plan. Both have also signed up to cover each other and are enrolled in the same plan. Our vendor is not coordinating benefits for these individuals. Is there a Medicare rule which prohibits individuals from being enrolled in two Medicare plans? I am just a little confused. The vendor coordinated benefits earlier in 2006, then stopped mid-year. This leads me to believe that the vendor thought COB was okay (at least originally). Any help/guidance will be appreciated!!