I found the solution for this in another blog... Used replace(Param.value, "&[", "%2526[") instead of escape() and it worked 🙂

="void(window.open('"+Variables!ReportServerUrl.Value+"/Pages/ReportViewer.aspx?"+Globals!ReportFolder+"/Test+Report&rs:Command=Render&Param1='" + replace(Fields!Realname.Value, "&[", "%2526[") + "&Param2=" + replace(Fields!Username.Value, "&[", "%2526["),'_blank'))"