They actually dont have to be the same. This has been tested by Jack Ricard from EVTV. My numbers might not be exact but roughly, he connected 1 Tesla 85kwh pack that was 90% charged to another Tesla 85kwh pack that was 30%. The packs shut down. It was too much current.
So he got them a little closer, one was 90% and the other was around 40% and it worked! The peak current was only 255 amps (nothing for a Tesla pack). They balanced out after about an hour.
So it can be done.
Since posting this I completed my range extending battery pack. I have 2 Tesla Rav4 EV packs running in parallel. Here is my youtube page. https://www.youtube.com/user/jimbo69ny