Play card before previous player has finished its turn
If you look at the rules:


  Draw Two – When a person places this card, the next player will have
  to pick up two cards and forfeit his/her turn. It can only be played
  on a card that matches by color, or on another Draw Two. If turned up
  at the beginning of play, the first player draws two cards and gets
  skipped.


You can not play a second draw-2. Although this seems to be a common house rule. 

Then again, it is nice to wait until the player has drawn the two cards before playing the next card.