I was referring specifically to combining several interfaces of the same type into a single virtual device, eg. using three 8-channel interfaces as a single 24-channel device, or a couple of 64-channel MADI interfaces as a single 128-channel device.
JACK has no trouble working with multiple devices as long as you only want to use one at a time. It can use different devices for capture and playback, but it can't combine several hardware devices into a single virtual device as I described above. That's what the pcm_multi plugin in ALSA is for (and as you mentioned, hardware clock sync is required).