Yep! Linux + Jack supports a single sound card. The reason is that the sound subsystem must sync to the device’s clock and each device will have a different clock. Some professional soundcards costing many hundreds / thousands of pounds support slaving their clock to another, e.g. using word clock or links between cards but these would need to present as a single device to allow jack to use it as its master clock.
There are some solutions which allow multiple sound cards to hook into jack, like alsa_in and alsa_out which connect to an audio device and connect it to jack by applying samplerate conversion. Indeed we use this to implement the headphone output via the Raspberry Pi onboard audio interface but it adds a significant load to system resources and can trigger xruns. It is a neat idea but not recommended for performance.
You could add a feature request to allow additional audio interfaces to be configured but I doubt it will be high on the priority list (unless there is a sudden realisation of its substantial worth).