Please look at this picture
It contains two players, Torres and Suarez.
These both were players for Liverpool football club. However, they were not playing at the same time. In other words, they have never played for Liverpool together.
Today there was a charity game in Liverpool and they played together in the same team.
Today the "Daily Mail Sport" magazine states in this link
These two are on at Anfield. What a duo they would have been for Liverpool FC. More here: Daily Mail
It says "would have been".
My question is why not could have been?
I think "could have been" is more correct because it is trying to say that if no one leaves Liverpool, they could have formed a good partners.