twenty ten- different image for header and post thumbnail?

Hi,

I’m setting up a new website which will be based on Twenty Ten.
I’m having this issue:

On hand, I’d like to keep the function where the header image in a page\post shows the “featured image” when they match in size.
On the other hand, I would like to use conventional post thumbnails (3:4 aspect ratio ot similar).
I know that when enabling post thumbnails, I should use again the “featured image”.

So how can I set up working post thumbnails and also keep the custom header image function going?

Thanks,

Tal