I was under the impressoin that if you hooked up the Matrice 300 to a second screen that the screen on the controller shuts off. But I connected the controller to my laptop via the hdmi port and both screens worked.  Does the screen only shut off when you connect a second screen to it via the usb port?

I have used an Android tablet via the USB port and all display related functions shift to the tablet, as in touch screen, apps, WiFi, Broadband Internet, everything. I'm having trouble getting my tablet to work due to some version differences where the Android Pilot App doesn't support the P1 Camera, but it does work. To be honest I can't remember if the display on the R/C turned off while I had the tablet in place, but it probably did.
The buttons and knobs on the R/C still work, but all screen interface functions shift to the tablet.

The HDMI port is just a way to export the R/C display to another screen, like a TV. One way - out. No control or touchscreen operations shift to the new device via the HDMI

Hope this helps

Yes. Screen shuts off when you connect an android tablet through USB. Screen stays on when HDMI is being used.