In the traditional rules of UNO, you cannot stack cards. So, if someone plays a Draw Two or a Wild Draw Four card, the next player must draw two or draw four cards. They can’t play a Draw Two or a Wild Draw Four card on top of the other card and pass the consequences of those cards down to the next player.
However, there are many house rules and variations that people play with, and some of these involve stacking cards. For example, the Stacking Draw Two or Stacking Wild Draw Four house rules usually mean if a player plays a Draw Two or Wild Draw Four card, the next player can avoid drawing cards by playing a Draw Two or Wild Draw Four card of their own.
In this case, the number of cards to be drawn accumulates and the next player in line who can’t play a similar card has to draw the total accumulated cards. This house rule can lead to more strategic and exciting games, but it’s not part of the official UNO rules.