Is it possible to detect the signal strength of a connection between two iOS devices by using Multipeer Connectivity Framework?
Unfortunately, it's not possible. I think there is a good reason for this. As you can read in the docs:
The Multipeer Connectivity framework provides support for discovering services provided by nearby iOS devices using infrastructure Wi-Fi networks, peer-to-peer Wi-Fi, and Bluetooth personal area networks and subsequently communicating with those services by sending message-based data, streaming data, and resources (such as files).
In case of infrastructure Wi-Fi network, there is no direct connection between two iOS devices as they communicate through an access point. Therefore you can't get the signal strength of a connection, because it simply doesn't exist.
If you want to get RSSI, you can use Core Bluetooth framework instead.