It is supposed to happen automatically.
I don't really know what I'm talking about on this one, but here are the first steps I'd take in troubleshooting.
Go to the iBooks Store on both devices and make sure you are logged in with the same account. You should be able to see this at the bottom of the page.
Go to Settings >> iBooks, and make sure Sync Bookmarks and Sync Collection are turned on. I'm not sure they have anything to do with it, but it's worth a try.
Make sure you have an internet connection before opening the book. It may sync on opening, and not in the background.
And that's all I can think of, at last at the moment.
Addendum:
I just fired up my iPod Touch (which I haven't done for several weeks) and tried a book I'd never opened on it before. At first it opened on the first page, but after a few seconds went to the last page I'd been on with the iPad.
That suggests two things to me.
One, the device must be connected to the internet when opening the book to go to the page.
Two, being signed on to the same account inside the app is probably important.