You would be able to accomplish this using the UA221. https://www.bhphotovideo.com/c/product/1335931-REG/shure_ua221_rsma_reverse_sma_passive_antenna.html

The Shure UA874 Active Directional Antennas should be separated by a minimum of one-quarter wavelength to ensure adequate diversity performance, which is about 4 inches for UHF frequencies. The diversity reception can then be improved by separating the antennas further to ensure line-of-sight with at least one of the antennas from any location in the room.

They can be mounted permanently or used in a mobile kit. They are large paddles and improve the pickup of the wireless frequency being broadcast by the mics. We have a set at our church permanently and I have a set in a Shure wireless 4 pack I rent for productions work.

No, in the situation as you describe the antenna is not necessary.

The item pictured is the US version 470-688. The item you are selling is the WB 470-900. this is confusing to your customers. Change the picture.

This antenna can receive signal from any number of active transmitters within range (based on the antenna's position and gain settings) that are transmitting between 470698 MHz. To clarify, this item's purpose in your wireless setup is to act as an extension of your receivers which, when properly placed and configured according to your application, can improve your transmitter(s) signal reception. However, it should be noted that your receivers are the devices that are actually connecting (or syncing) with your transmitters to interpret the captured signal of each active wireless microphone or bodypack. Thus, any limitations in number of transmitters you are able to use simultaneously would be based on the capabilities and coordination of your receivers rather than the number or type of antennas you are using.

This would depend on whether your specific application calls for a diversity or non-diversity wireless system and what kind of coverage is needed. In many circumstances however, using two is most common.