To solve the mystery of how you can get gain out of what might appear to be a omni-directional whip: This is a collinear antenna which has coils built into it so that it acts like several vertical antennas are stacked on top of one another, each creating more gain as the radiation pattern is squashed down into the shape of a toroid or doughnut so that little power is received or transmitted either directly above or below the antenna and instead concentrated in a band around the center.
If the Wi-Fi antenna you want to receive from is elevated above you then you would want this kind of a antenna to be swiveled in such a manner as to put the whip at a 90 degree orientation to the remote station to allow signal reception or transmission off of the sides of the whip where the power is greatest.
Due to the way the receive and transmit pattern is in the shape of a toroid or doughnut this type of antenna can give good gain while at the same time being omni-directional, except above and below the antenna where you don't need to waste the power anyway.
If you want a antenna which has much higher gain look for a more directional antenna such as the flat panel antennas, Yagi or a parabolic reflector type of antenna which depending upon the size, can have 20 dB (or more) of gain which is a factor of 100 making a 100 milliwatt or 1/10 of a watt signal seem as powerful as a 10 watt signal, or a 1 watt signal as powerful as 100 watts. If you buy a flat panel Wi-Fi antenna and get one rated for 20 dBi of gain the half power beam width for both receive and transmit will be about 20 degrees.
With a 20 dBi gain flat panel directional antenna, if you point it 10 degrees to the left or right from where it should be pointed for maximum signal you will have the equivalent of 17 dB of gain but that apparent gain will quickly drop off the further away the antenna is pointed from the proper direction. Note: It is a coincidence that a 20 dBi gain antenna has a 20 degree wide half power beamwidth, normally gain and half power beamwidth do not correlate to the same numbers except in the case of a 20 dBi gain antenna.
If you want to receive or transmit to two different locations at the same time but want each to have the best signal possible using a single high gain flat panel antenna, you can cut the difference between them sacrificing some of the gain to each one and point the antenna to an area in the middle. As long as the two stations are close enough together to be within a healthy portion of the antennas area of gain this should work out fine.
Obviously, you wouldn't expect to be able to use that antenna for two different locations if one was behind you and the other in front but if both are within 30 degrees from you and not too far away a 20 dBi gain antenna could be used by cutting the difference between them. If they are too far separated in azimuth direction from you use a high gain collinear antenna with more power on each end, or perhaps using a lower gain antenna which has a much wider beamwidth or separate antennas for each radio shot.
You can also use a splitter designed for 2.4 GHz and combine the receive and transmit from two separate antennas together into one feedline going back to your equipment, if necessary, but there will be a 3+ dB penalty or loss of signal level when doing so but to me this seems like the best solution if you need all the gain you can get from a flat panel or highly directional antenna to each station when using a common or single Wi-Fi transmitter and receiver such as a LinkSys or other wireless router.
Here is a good rule of thumb for line of sight radio shots without buildings or other terrain in between, every time you half the distance you increase the received power from a remote location by six dB, or if you double the distance you lose six more dB of signal level. In areas where there are trees and buildings or hills between each location this rule does not apply as the signal losses can be very great for short distances.