Casio definitely thought about the student/teacher relationship when designing the PX-160. It has a duet play mode, which splits the keyboard into two halves. Each half is the same octave range (about three and a half octaves per side), so a teacher can demonstrate on one side while the student plays on the other at the same pitch. Two headphone jacks allow both players to use headphones without the need for a splitter. If the speakers are being used, they can also be configured to output sound from the left keyboard side to the left speaker and vice versa.
Maybe you've already got a specific brand in mind and want to see all the digital pianos they offer. All you have to do is click on the brand on the left hand side of the screen and within a few moments you'll have access to everything that's available. For that matter, you can always play is safe and stick to the top-rated items…and there are plenty of those in this selection.