Assuming that you think you'll get a boost over a single yagi, it won't do anything for you. Sorry. It shows you're thinking, though!

Too bad, it would have looked cool.

Dual antenna designs are made to reduce problems associated with the Receive signals bouncing off nearby objects (which they do at these frequencies) and arriving slightly out of sync with each other. The problem is called "multipath." A good anology is "ghosting" on a TV picture; same type of effect. These duel antennae are configured in a switching system usually called "antenna diversity." Using diversity switching, the strongest signal is prefered over the weaker. On Transmit, only on antenna is used.

If you used two yagis in a side by side "shotgun" setup, one of two things would happen: 1) the switch would lock on one yagi, and only one would ever be used; or 2) the switch would continuously oscillate between the two yagis.

To use a yagi with these kinds of AP, you put the yagi on one port, and turn the other antenna off, using the software controls.

Search on "multipath", as its been discussed in these forums before.

Cheers,
Thorn