Hi Pierre,
Thanks for using one of my Flickr photos to start this discussion thread.
You asked: Do you need 2 extra screens?
Answer: No, these screens are optional, depending on your needs.
The 21.5-inch Dell S2240T touchscreen monitor resting on the Genos music stand has VGA, DVI, and HDMI inputs. I connected a Plugable USB 2.0 video display adapter to the back USB 2.0 port of Genos to mirror its display to the Dell monitor through HDMI. I also connected the same Dell monitor through DVI to HDMI adapter to a Windows 10 Pro laptop so I can run programs that enhance the operations of Genos, such as YEM, YRM, StyleMagic, StyleExtractor, StyleFinder etc. right in front of the Genos with a stylus, and a Logitech bluetooth keyboard. I can switch the Dell monitor to display the Genos screen, or the laptop screen any time I want.
The ipad Pro 12.9 is for SongBook+ music score / lyrics display mainly, connected to Genos through USB3.0 camera kit, or bluetooth wireless MIDI through Yamaha MD-BT01 bluetooth wireless MIDI adapter plugged at the back of Genos for registration / song synchronization.
You asked: Is it possible to use Ipad as a monitor as well?
Answer: Yes and No. Yes if you want to spend thousands of dollars to buy Teradek wireless video streaming devices to stream the Genos display signal to your ipad. Most people, including me, will not spend so much money to make an ipad a remote video display.
If you use a Windows 2-in-1 touchscreen laptop or a MacBook Pro instead of an ipad, you can purchase a $129 Elgato Cam Link USB 3.0 video capture stick to get HDMI input and display the Genos screen on the laptop screen at very low cost.
You asked: Or does Ipad only work with Yamaha's apps?
Answer: No, the ipad / ipad pro works with apps from Yamaha such as Chord Tracker (see an extensive list of Yamaha ios apps
https://usa.yamaha.com/products/apps/index.html), 3rd party apps such as BauM Songbook+ and many more.
Thanks,
Paul