How can I receive 2 video streams from 2 Android phones over USB tethering using IP Webcam?
USB tethering is not necessary, just use adb port forwarding. It allows you to connect to  multiple devices via USB. On the PC you need Android Debug Bridge from Android SDK installed. On OSX the simplest way should be using brew and install android-platform-tools.
And on all Android device the development menu has to be enabled and the Debug Bridge mode has to be activated.
First get a list of connected devices via to get the device ID of each device.
adb devices

Example output with the two devices HABY39RL and DARY42EE:
List of devices attached
HABY39RL        device
DARY42EE        device

Next establish a port forwarding for each device:
adb -s &lt;device ID 1&gt; forward tcp:8080 tcp:8080
adb -s &lt;device ID 2&gt; forward tcp:8081 tcp:8080

Afterwards you can connect to the web cam app on the first device on the PC using the address localhost:8080 and on the second device using localhost:8081.