You can indeed have 2 auto-nav blocks. What you're going to do is specify which pages you want to show up in your auto-nav when you add the block. I'm guessing the "sub-nav" will be 2nd or 3rd level pages and your first auto-nav will be top level pages. Either way, you will select which pages to show up in each when you add them.
In order to add hover effects to your auto-nav, you can simply add css selectors such as

thanks for the hints...
just one more question:
i just entered the second auto nav. but i can not get it in the center of my header block.
i just can´t find the right css code...
thanks!

As long as its already in the block, you may want to set it from the front end by selecting "Design" when choosing to edit it. Select "alignment" under the Font tab of the editor

thanks again...
first it didn´t work. the i went throught my main.css file and got rid of a few things i just put in before... so now it works perfectly!
thanks for the fast and good help!