As mentioned there is a company called ON STAGE that do a budget priced Z stand with tier for a second keyboard.
http://www.onstagestands.com/catalog/item/home/model/ks7365-ej/grp/GRP000072You could also try Bespeco:
http://www.bespeco.it/catalogo/prodotti....0&lingua=enBut by far the best Z stand I have found is the Z-70 by Quiklok - build quality is superb but are not what you would call cheap - stability and quality comes at a cost! We use these with our Audya's at exhibitions and in the showroom.
http://www.quiklok.com/catalog/?p=p_3We do not use X stands due to safety and stability.