Quote Originally Posted by HDNitehawk View Post

The 600mm is a bit bigger in diameter. Also you will not get a sense of how the 600mm actually balances, and that is the most important aspect of handholding such a long lens. The farther you move your hand from your body that is holding the lens the harder it will be to hand hold.

I have tried the 2x on my 500mm, the balance point changes and it is much harder to hold than the 500mm without it.
That's why I thought by adding the 2X to the 500mm I, it would make it longer and harder to hold to simulate the 600mmII.