The Master combinations in Dr Who the Card Game
Based on the rulebook it stated there 


  You can add Enemy cards to one of your attacks, as long as the card
  or cards you play can be legally played with the cards already in
  place


Based on my understanding, if The Master is already in play along with another enemy, the enemy card you are going to add should be legally played with the cards. (meaning all of the cards) including the other enemy cards with The Master.