You can set the AP to use one or two antennas in the interface configuration.
Dual antennas are usually used inside in "Diversity" setting: Diversity is a mechanism to reduce the effect of multipath from signals that are bounced around inside.
Diversity "listens" on one antenna, then the other, never both at the same time. They are spaced such that a flakey signal on one antenna is likely to be improved on the other.
For most external applications (like with the Yagi) you only use one antenna.
Were to mount the external antenna will depend on the range between systems, clear line-of-sight, and geography ... you put the antennas high enough to clear all interfering foliage or structures (plus some for the "fresnel zone."
The other thing to remember is "environmental events:" there are things called "Lightning arrestors" that ae NOT effective against direct or near strikes ... they;ll bleed static electricity are reduce your chances of getting hit, but if it gets hit, your system is toast.
A much better guard against internal system damage is to use a segment of fiber optic cabling between the AP/bridge and the internal LAN to electrically isolate the external systems from the internal systems.
There are a number of vendors for copper --> fiber / fiber / copper transceivers for well under US$100.00 each side.
Good Luck
Scott
