Two quickdraws are the standard, locking biners if you want to be really safe.  The self-equalizing action of the "magic X" also leads to extension should one anchor point fail, this is understood to be a bad thing.  Of course if the anchors are as bomber as they should be, you'd expect your gear to fail first, and even this is highly unlikely in a toprope situation unless there is unusual crossloading or rock contact on your gear. 
  
Specifically regarding your partner's setup, the problem with that is there is only one sling holding the setup together, so if anything goes wrong with that one sling, there is nothing backing it up.  Due to the fact that is in the "magic X" configuration and not tied off at the attachment point, failure at any point in the sling will eliminate your anchor.