UNO Stacking Wild Draw 4 Cards?
According to official uno rules:


  Wild Draw Four – This acts just like the wild card except that the
  next player also has to draw four cards as well as forfeit his/her
  turn.


Emphasis mine.

This means player 2 already made an illegal play according to the official rules and you are playing under house rules. As you are playing house rules there is no official answer.

However when it comes to challenging the rules state:


  With this card, you must have no other alternative cards to play that matches the color of the card previously played. If you play this card illegally, you may be challenged by the other player to show your hand to him/her.


The other player here being the next player.

This means that a challenge is always against the previous player, in this instance player 2.

In summary. A challenge is always against the previous player, in this case player 2. However as you are already using house rules by allowing WD4 on a WD4, you can also  make it a house rule that player 1 is challenged in this case.