That seems to contradict even my admittedly basic understanding of how digital audio devices work. Mixing unsynced devices for simultaneous I/O is asking for trouble. JACK has no inbuilt capacity for synchronizing devices, so you will get clock drift and as far as I know potential sample misalignment between what you are listening to and what you are recording. Using a single device with the same clock for both input and output is always preferable, followed by two devices with some kind of hard sync if possible, followed by an aggregate driver level (ALSA in Linux) device only if the other options are unavailable. I've probably made a mess of explaining this, but there are multiple articles on the web about the importance of clock sync if you want to read up on it.