Same reason as for the receivers used with professional quality wireless microphones - their receivers typically have twin antennas.

I'm sure you've noticed that when listening to FM in your car, there will be locations where you need to pull ahead a few inches to have good reception. In fact, the process becomes nearly automatic where you always leave a slightly larger space in front of your car at those intersections in case you need to roll forward a quarter wavelength.

Twin antennas provides similar 'space diversity' to avoid nulls.  If one antenna is in a null, then the other won;t be.

Thanks for the info.  I understand.  But does that mean I can use either port for the external directional antenna?

If everything is as I think it is, then you should be able to use either port.

Leave a whip antenna on the second port just in case it helps.  In any case, it would likely be better than leaving the second port open circuit.

If you run into any difficulties, then swap ports (directional <-> whip).

I wonder how much antenna gain I would need to tap into the free WAN that is about 10km up the road...

It is for receiver diversity--Most of the systems will have two little antennas on the mobile device and two on the wireless router.  The simplest use is to sample the signal from each receiver antenna and then quickly decide which antenna has a stronger signal before data comes in the packet structure.  The high frequencies (2.4 or 5.8 Ghz) indoors will allow a lot of multipath and the desired signal may be accompanied by a bounced signal that partially cancels the desired by adding out of phase with the carrier (a fade).  Having two antennas separated helps because both antennas being in a fade will be less likely.

Regarding Antenna gain for reception 10 km away.
Wireless communication range/distance is reduced often by a factor of 2-6 due to multipath and normal range calculations get difficult/impossible.

Assuming that your wireless communication doesn't have multipath and is 500 feet with normal zero dBi antenna gains, everytime you double your distance you need 6 dB (factor of 4) more antenna gain in your receiver to keep the same signal strength. 10 km is 6 miles = approx. 30,000 feet. So to see how much extra gain you need, here is some basic math.

dist (feet) vs. antenna gain (dBi)
500   = zero (original antenna having 500 foot range)
1000  = +6
2000  = +12
4000  = +18
8000  = +24
16000 = +30
32000 = +36 dBi gain or 4000 gain.
Gain = 40,000/[(beamwidth azimuth) x (beamwidth elevation)]
or beamwidth = sqrt(40,000/4000)= sqrt(10)= 3.3 degrees.
at 2.4 ghz wavelength = 11.803"/2.4 = 5 inches.
3.3 degree beamwidth is approx. 20 wavelengths, so your antenna would be about (20x5")or 100" x 100" in size. That's pretty big.
KCH.