The 3.5 inch touch display of the streamer itself goes via gpio pins + touch plug-in required.
and the other raspberry pi 3 (which I use as a server) via HDMI TV.
if you use dsi with the original 7 inch raspberry pi touchscreen then you just need to install the touch plug-in.
I don’t know if the raspberry pi 4 can be used 2x hdmi at the same time.