For a device like the nRF9160 Feather, you need a second USB to UART device for listening to the modem traces. When I go to select the Terminal Serial Port under Advanced Options, it only gives me one option (the external FTDI adapter). As far as I understand, they can't be mapped to the same interface.
Running on Linux. Will try on Mac to double check it's not a platform specific issue.
On Mac I'm able to select the separate ports but get errors:
11:10:14.681 Attempted to retrieve trace databases for an unrecognized device {"id":1,"serialNumber":"78b596379247ec1189dadbde461fcfc8","serialPorts":[{"comName":"/dev/tty.usbserial-14510","manufacturer":"Silicon Labs","path":"/dev/tty.usbserial-14510","productId":"EA60","serialNumber":"78b596379247ec1189dadbde461fcfc8","vcom":0,"vendorId":"10C4"},{"comName":"/dev/tty.SLAB_USBtoUART","manufacturer":"Silicon Labs","path":"/dev/tty.SLAB_USBtoUART","productId":"EA60","serialNumber":"78b596379247ec1189dadbde461fcfc8","vcom":1,"vendorId":"10C4"}],"traits":{"boardController":false,"broken":false,"jlink":false,"mcuBoot":false,"modem":false,"nordicDfu":false,"nordicUsb":false,"seggerUsb":false,"serialPorts":true,"usb":true},"usb":{"device":{"address":43,"busNumber":0,"configList":{"descriptors":[{"bDescriptorType":2}],"interfaceLists":[{"descriptors":[{"bDescriptorType":4,"bInterfaceClass":255,"bInterfaceProtocol":0,"bInterfaceSubClass":0}],"endpointLists":[{"descriptors":[{"bDescriptorType":5},{"bDescriptorType":5}],"length":2}]}],"length":1},"descriptor":{"bDescriptorType":1,"bcdDevice":256,"idProduct":60000,"idVendor":4292}},"manufacturer":"Silicon Labs","product":"CP2102N USB to UART Bridge Controller","serialNumber":"78b596379247ec1189dadbde461fcfc8"},"serialport":{"comName":"/dev/tty.usbserial-14510","manufacturer":"Silicon Labs","path":"/dev/tty.usbserial-14510","productId":"EA60","serialNumber":"78b596379247ec1189dadbde461fcfc8","vcom":0,"vendorId":"10C4"},"favorite":false,"nickname":""}
11:10:14.681 Attempted to retrieve trace databases for an unrecognized device {"id":1,"serialNumber":"78b596379247ec1189dadbde461fcfc8","serialPorts":[{"comName":"/dev/tty.usbserial-14510","manufacturer":"Silicon Labs","path":"/dev/tty.usbserial-14510","productId":"EA60","serialNumber":"78b596379247ec1189dadbde461fcfc8","vcom":0,"vendorId":"10C4"},{"comName":"/dev/tty.SLAB_USBtoUART","manufacturer":"Silicon Labs","path":"/dev/tty.SLAB_USBtoUART","productId":"EA60","serialNumber":"78b596379247ec1189dadbde461fcfc8","vcom":1,"vendorId":"10C4"}],"traits":{"boardController":false,"broken":false,"jlink":false,"mcuBoot":false,"modem":false,"nordicDfu":false,"nordicUsb":false,"seggerUsb":false,"serialPorts":true,"usb":true},"usb":{"device":{"address":43,"busNumber":0,"configList":{"descriptors":[{"bDescriptorType":2}],"interfaceLists":[{"descriptors":[{"bDescriptorType":4,"bInterfaceClass":255,"bInterfaceProtocol":0,"bInterfaceSubClass":0}],"endpointLists":[{"descriptors":[{"bDescriptorType":5},{"bDescriptorType":5}],"length":2}]}],"length":1},"descriptor":{"bDescriptorType":1,"bcdDevice":256,"idProduct":60000,"idVendor":4292}},"manufacturer":"Silicon Labs","product":"CP2102N USB to UART Bridge Controller","serialNumber":"78b596379247ec1189dadbde461fcfc8"},"serialport":{"comName":"/dev/tty.usbserial-14510","manufacturer":"Silicon Labs","path":"/dev/tty.usbserial-14510","productId":"EA60","serialNumber":"78b596379247ec1189dadbde461fcfc8","vcom":0,"vendorId":"10C4"},"favorite":false,"nickname":""}
11:10:22.855 Failed to connect to port: /dev/tty.SLAB_USBtoUART. Make sure the port is not already taken, if you are not sure, try to power cycle the device and try to connect again.
11:10:22.855 Terminal Serial Port: Port could not be opened. Verify it is not used by some other applications
Despite what it says, I'm not using the port anywhere else. Seems like it may bind/connect to the the interface to the "Modem trace serial port" despite changing the setting.
If I try to use the other interface, then I don't get an option in the "Terminal Serial Port" to use the SiLabs part:
So it may not be platform dependent.
I just ran into this myself. I can setup my monitor client firmware(Rust) to send traces at 1M or be used as an AT command driver at 115,200 baud but not both. 
Using the Conexio Stratus dev kit.
Do the supported devices really have two separate uart bridges?