It's going to be the ability to act as an AP and a client at the same time. Frequently this means two physical devices.
Dual wifi makes for easier installation/operation in a lot of situations though, so I think its a good "default config". If there's already a wifi network people can use, hooking one wifi chipset to that then connecting the phones under test to an access point broadcast by the other - is easier than expecting there to be a live ethernet port and someone at a womens shelter-style-place having access to it and skills to use it.
This should be fairly simple too so long as there's no arbitrary restriction on requiring wlan devices preventing you from using WiFi+Ethernet rather than Wifi+Wifi.