Use scrcpy to connect to multiple smartphones
If multiple devices are connected you can select the device to connect to by specify it's serial number. This is documented on the scrcpy help page. Therefore you can start one scrcpy instance per device.

Multi-devices
If several devices are listed in adb devices, you must specify the
serial:

scrcpy --serial 0123456789abcdef 
scrcpy -s 0123456789abcdef # short