As far as I am aware they were made to leave the field because they were not injured in the same incident. For instance if the two players collided into each other then they would not be required to leave the field.
The referee will not require two players from the same team to leave the field if they collide and they need immediate attention (ie. are seriously injured).
p. 73 - 74 of the 2013/14 FIFA Laws of the Game (abridged for clarity):
The referee must adhere to the following procedure when dealing with injured players:
- play is allowed to continue until the ball is out of play if a player is, in the opinion of the referee, only slightly injured
- play is stopped if, in the opinion of the referee, a player is seriously injured
- as soon as the referee has authorised the doctors to enter the field of play, the player must leave the field of play, either on a stretcher or on foot. If a player does not comply, he must be cautioned for unsporting behaviour
- an injured player may only return to the field of play after the match has restarted
Exceptions to this ruling are to be made only when:
- a goalkeeper is injured
- a goalkeeper and an outfield player have collided and need immediate attention
- players from the same team have collided and need immediate attention
- a severe injury has occurred, e.g. swallowed tongue, concussion, broken leg