Umm not really sure either but here is a quick video of what I can understand you are trying to do.

from the screenshot it looks like you have just one object but perhaps you need to separate them like I do at one point in the video and then you can set one of the objects as the driving force and cause the other to become the driven by setting specific animation keys but if this is for a game I doubt that is the route you are wanting to take.

https://screencast.com/t/c1VppFRb