Keeping Books On My 2 Kindles, Version 2 And Dx?

I have a Kindle 2 which is small and perfect for reading in bed or while traveling, I also just bought a DX version for during the day when a larger screen would be helpful.
I have 400 books on my Kindle 2, I want to keep them on it, is there a way of loading these books onto the DX and keeping copies on the Kindle 2 at the same time ?
Both versions are for my own personal use..
Thanks for your time..