At least for the DIGI manufacturer, It is stated for the PRO-XSC variant of the Xbee module a maximum outdoor achievable distance of 14Km w/ passive antenna, nevertheless supposedly for direct view. For this case in particular, in theory you could just use one module as transmitter and another as receiver. Even so, it is necessary to do a procedure commonly called site survey on wich the link performance is measured in terms of local interference and signal strengh.