In order to achieve 300 Mbps raw data link rates on outdoor 802.11n line-of-sight bridge links, polarize one antenna chain horizontally and one antenna chain vertically.
Due to the use of MIMO (multiple input, multiple output) in 802.11n, multiple spatial streams are required in the same time domain. Indoor networks rely on sufficient multipath in the environment in order for the receiver to distinguish separate data streams. Outdoor bridge links cannot provide sufficient multipath in order accommodate multiple independent spatial streams due to fundamental differences in design to prevent fresnal zone blockage and to ensure link stability and reliability. Thus, they require opposite antenna polarization to sufficiently distinguish the streams at the receiver.
Theoretically, two spatial streams "should" be the maximum supported for outdoor point-to-point bridge operation. But with the innovation in advanced antenna design being done by many companies today (Ruckus, MPAntenna, etc.), I think someone is bound to find a way to support 3 or possibly even 4 stream 802.11n on outdoor links.