Second Macbook Pro as extended display I have an old Macbook Pro 15". My company gave me a new Macbook Pro 13".

Question, is there a way to use either of them as extended display to the other one ?

Both support retina display.
There is no built-in Apple way to do this. Your only option would be to use third party software such as Air Display. However, I cannot vouch for how well it will work in your scenario.