I haven't seen an option to do that, specifically.
But, the way I got round it was by doing thus:
Simply get your desktop picture, and in Photoshop or something, make the width double the size. In the space you have, add another picture, so you have two pictures side by side. (So 1 picture fills one monitors width in res, while the other fills the other monitors width in res)
So for a 1280x1024 resoloution on both, you'd have a 2560x1024 res picture.
When you then apply this desktop image, you have two seperate images on different screens.
I hope this helps