Not sure why you would actually want to fiddle with mattes beyond your primary keying/ mask. Any DOF is an illsuion in such a scenario and best controlled by using separate effects on ther layers involved. If you have a particular reason why you want to use Lensblur with an input buffer, perhaps a screenshot might help to explain...
I am not sure how to separate the layers.
I have a subject in the foreground that I need to isolate into a separate layer and I am not sure how to do that considering the person is moving.
I am not using the right terminology In video land we used to call it a traveling matte.
I can't post the a pict because it is not my material. Will look for something similiar.
Masking? Rotsoscoping? I can think of a million ways to create separated layers or mattes and then duplicate the layer and invert the masks/ matte, but if you never have used that stuff before, you will hard-crash on concrete and not get very far, I'm afraid. It takes at least a bit of practice to produce a clean matte. As a palce to start you may get by suing the Rotobrush and see, if it produces clean enough edges, but if not, it's proably better to scrap the idea before eitehr ending up with a bad result or not meeting your deadline...