The TR-004 is seen as an external drive in QTS. If the NAS has several USB inputs, you can put one per input (in any case 2 units according to the TR-004 User Manual). I haven't tried this myself, but I don't see the limitation.
Choose a USB 3.2 port.
I connected mine with software management and only for backup (not as an extension to the raid storage pool).

Perfect thanks. I have read that this should not work with two devices despite Qnap's specification.