A video and lyrics are both considered "documents" and you can't show two documents on the same screen at the same time. If you want to show the video and the lyrics at the same time, you'd have to connect your device to a second screen. This can either be with a wire like an HDMI cable or wirelessly like with an AppleTV. Alternatively, if you have a second iOS or Android device, you can use the screen sharing option. Then change Settings > General Settings > External Display to Doc +1 to show a different document on each screen.