Airhead got it almost entirely right:
However, you don't need to spend the $ for it. Sveasoft distributes Satori firmware freely, and it will do exactly what is desired, in three different ways.
In the first two cases one unit has to be an AP. The second unit can be either a repeater or a client. If it is a repeater then the connection to it must be via a wireless client. If it is a client, then no other wireless connection can be made with it, and an ethernet link is required (but the AP can still make multiple connections).
Another way to do it is to run both units as clients in an Ad-Hoc mode, which is essentially a point-to-point link for both ends and allows no other wireless connections to either unit.
The Satori firmware has a great deal of other functionality, and is well worth downloading. Sveasoft's aggressive marketing of a beta version may or may not be distasteful to any given individual, and if it is not perhaps purchasing the latest release if worth doing, or not; and if Sveasoft is indeed deemed distasteful in their business practices, perhaps downloading and using their free version is an appropriate way to thumb your nose at the practices you don't like!