Hi rhed,
This is currently not a possibility through the Auctiva lister page. We will be adding this capability to our site following our Phase II release, which is tentatively scheduled for Q1 of 2007. In the mean time, in order to accomplish this, you would have to revise the items through eBay's "Revise your Item" form after thay have been posted through Auctiva, to add that you charge sales tax to two states.
-Mike