I’ve made a video tutorial on how to do this now: https://youtu.be/8ILaDz3F4wo
Otherwise, here are the written details:
With 2 objects the same size (front object & back object):
Select the front object. In the Animations Panel:
1. Name the object FRONT
2. Set the Preset to Shrink
3. For Event, uncheck On Page Load so it says Choose
4. Set Duration to .25 seconds (or longer if you prefer)
5. Keep Speed at Preset and make sure origin point is at the center of the object
6. Set Animate: From Current Appearance
7. Unlink Scale constrain button and set W to 0% and H to 100%
Now select the back object
8. Name the object BACK
9. Set the Preset to Grow
10. Set Event, uncheck On Page Load so it says Choose
11. Set Duration to .25 seconds (or longer if you prefer)
12. Set Animate: To Current Appearance (not From Current Appearance!)
13. Unlink Scale constraints and set W to 0% and H to 100%
14. For Visibility: check Hide until animated
Again select the object named FRONT
15. In the Buttons and Forms Panel, select Button
16. Name the Button FRONT
17. Set Event to On Roll Over
18. For Actions: choose Animation
19. For Animation: choose FRONT
20. For Options: choose Play
21. Add another Action: again choose Animation
22. Make sure Animation: is set to BACK in the dropdown list
23. For Options: choose Play
Now select the object named BACK
24. In the Buttons and Forms Panel, select Button
25. Name the Button BACK
26. Set Event to On Roll Off
27. For Actions: choose Animation
28. For Animation: choose Back
29. For Options: choose Reverse
30. Add another Action: again choose Animation
31. Make sure Animation: is set to FRONT
32. For Options: choose Reverse
33. Align the the two objects on top of each other
34. Done. Test with EPUB Interactivity Preview or Publish Online
