According to my understating i can configure two arcgis server with one web adaptor e.g my web adpator name is arcgislb
and two gis server are gisserver1.ad.net and gisserver2.ad.net. But the problem is if i add one other is replaced i.e. if i add gisserver2.ad.net after adding gisserver1.ad.net then gisserver1.ad.net is replaced by the seciond server.