playBunny: For the last time. But you don't seem to be able to grasp the concept that rules should be complete, and not refering to things that aren't defined.

• The rules of backgammon (as stated on THIS site, not rules of backgammon defined by some other identity) nowhere state that if it is possible to move with both die, you have to do so. (This is your MDU rule).
  
• Therefore, the "no swapping possible if there's no legal move for the second die" isn't referring to the MDU rule, because there is NO MDU rule.
  
• Considering that you have to pass if there is no legal move available (no my words - read the rules), and you call this situation "impossible moves", it seems that what you call "impossible moves" is what the rules call "no legal moves".
  
• Again, the rules do not define any MDU rule.
  

Now, either quote the specific part of the rules that define your MDU rule, or stop using arguments that there is an MDU rule.