Personally I see no reason why you cannot mix sterling silver and argentium silver in your jewelry designs. The only real difference is that the sterling will eventually tarnish black while the argentium will tarnish a pale blonde color. I often mix sterling silver beads with argentium wire and sometimes both wires as well. When you write the product description for these combined items, be sure to list both metals so your customer is aware of the uniqueness of their purchase.
Answer contributed by Dale `Cougar` Armstrong