One of my favorite piano apps is Real Piano 2 (free, with a $.99 (USD) in app upgrade to remove ads). You can set up to (1) have just one octave, (2) show four octaves - two on the top of the display and two below (all octaves move by swiping the title bar), or (3) the same four octaves as #2, but the top row and bottom row can be moved independently of each other.
I've looked a long time for a piano app. This is the one I am most satisfied with. Of course, and as always, YMMV.
Marilyn