[Jackaudio] Is it possible to connect two usb input devices to jack (linux)? (neophyte question)

I have two usb audio input devices that I'd like to mix and connect to Jamulus, an application that uses jack (not pulseaudio). I'm trying to find out if this is possible. I'm running jack2 on a debian 10 system. There's one audio card (chip, actually) that's listed as HDA Intel. FYI The usb input devices are a Shure MV5 microphone and a Scarlett Solo audio interface. I can connect either of these devices to jack by specify one from qjackctl setup: Settings -> Advanced -> Input device. I don't see how to capture and mix the input from both devices. I haven't found any documentation that explains how to do this or if it's possible. Thanks