Topics can only have one parent. So you must decide which forum should be the parent: x or y. There is no way to show the same topic in both x and y forums, unless one is a sub-forum of the other (threads from sub-forums are visible in the topics view of the parent forum).
~Tanveer~ wrote
or if its not possible could i merge the search engines on both forums
We don't have an easy way to do this. You may use Google Custom Search and add the domain of your two forums to the search, but you cannot add the search form to your forum yet because we don't let users add javascript to Nabble (this will change soon when we release the templates feature).