The only reason that the wands didn't work against eachother was because they both sent off spells and they met eachother in midair, and the wands connected. If and when Harry fights Voldy, it will only happen again if the spells meet it midair, again. Other than that, they can still cast spells on eachother, because if you will notice in the book Voldy uses the Cruciatus Curse and the Imperius curse against Harry without the wands connecting. (Chapter 34, Priori Incantatem, pg. 660 and 661) =^) Thats my say about anyways.