Perhaps you can explain why you want this. Unless I'm mistaken, the 'workspace' metaphor is specifically for another virtual space, not another physical space. For most applications, it's a benefit to be able to have your applications relate to each other.
In my experience, Gnome lets any app 'fullscreeen' app just expand to the size of the current monitor only, so the need need for separate X sessions is not now what it once was. But perhaps if you say a few words about your requirements someone might have the right idea.
If you really want separation, you may have to look into running a separate X server on each display (though the means will be different than described in thezachperson31's link). Does the idea of running two totally separate (and generally unable to easily communicate with each other) X servers serve your need?
On the other hand this question and answer seems to have filled the bill (and at a bounty price)