It seems that most DAWs have the same issue – my guitar is connected via my Yamaha THR10 (USB) – which records fine. However, I have a USB Rode microphone connected also – Soundbridge refuses point-blank to allow both of them to operate together. It seems I can only use one USB input. That seems bizarre. Renders it useless, unfortunately.
Each of your USB devices are registered as separate Audio devices.
SoundBridge only accepts one audio input device and one audio output device per time.
The only way to use two USB audio devices simultaneously is to aggregate them into a single audio device using an audio wrapper like VoiceMeeter Banana.