Can I use HDMI output alongside my LCD?
Yes and no. As explained in the official Pi blog on the subject, only applications which know how to output over HDMI can be used. An example is given for OMXPlayer: https://www.raspberrypi.org/blog/the-eagerly-awaited-raspberry-pi-display/2.4k
It is possible to use both display outputs at the same time, but it does require software to choose the right display. Omxplayer is one application that has been modified to enable secondary display output.
To start displaying a video onto the LCD display (assuming it is the default display) just type:
# omxplayer video.mkv
To start a second video onto the HDMI then:
# omxplayer --display=5 video.mkv
Please note, you may need to increase the amount of memory allocated to the GPU to 128MB if the videos are 1080P, adjust the gpu_mem value in config.txt for this. The Raspberry Pi headline figures are 1080P30 decode, so if you are using two 1080P clips it may not play correctly depending on the complexity of the videos.
Display numbers are:
- LCD: 4
- HDMI/TV: 5
- Auto-select non-default display: 6