This is an excellent question! I'm going to start with the requisite Site Survey recommendation (even for two AP's :)
You should try to perform a site survey to help determine the AP/Antenna types and placement.Make sure the site survey is done when the facility is in full activity mode and everything is up and running. Here is some info to help get you started.
"In a Wireless network, many issues can arise which can prevent the radio frequency (RF) signal from reaching all parts of the facility. Examples of RF issues include mulitpath distortion, hidden node problems, and near/far issues. In order to address these, you need to find the regions where these issues occur. A site survey helps you to do this. A site survey helps define the contours of RF coverage in a particular facility. It helps us to discover regions where mulitpath distortion can occur, areas where RF interference is high and find solutions to eliminate such issues. A site survey that determines the RF coverage area in a facility also helps to choose the number of Wireless devices that a firm needs to meet its business requirements.
A proper site survey provides detailed information that addresses coverage, interference sources, equipment placement, power considerations and wiring requirements.
These are some of the steps that are performed during your site survey:
Obtain a facility diagram in order to identify the potential radio frequency (RF) obstacles.
Visually inspect the facility to look for potential barriers or the propagation of RF signals and identify metal racks.
Identify user areas that are highly used and the ones that are not used.
Determine preliminary access point (AP) locations. These locations include the power and wired network access, cell coverage and overlap, channel selection, and mounting locations and antenna.
Perform the actual surveying in order to verify the AP location. Make sure to use the same AP model for the survey that is used in production. While the survey is performed, relocate APs as needed and re-test.
Document the findings. Record the locations and log of signal readings as well as data rates at outer boundaries."
An important concept to note regarding channel assignments is that the channel actually represents the center frequency that the transceiver within the radio and access point uses (e.g., 2.412 GHz for channel 1 and 2.417 GHz for channel 2). There is only 5 MHz separation between the center frequencies, and an 802.11b signal occupies approximately 30 MHz of the frequency spectrum. The signal falls within about 15 MHz of each side of the center frequency. As a result, an 802.11b signal overlaps with several adjacent channel frequencies. This leaves you with only three channels (channels 1, 6, and 11 for the U.S.) that you can use without causing interference between access points.