Trying to keep the display active on a RG35xx while casting to HDMI

I just picked up an RG35xx and I love using it! I tried to plug it into my HDMI screen so I could cast it to the bigger screen but still play on my original screen. The screen on the device goes black and the display is only on for the inputted device (the external screen). Is there a setting on my console to allow both screens to be on?