Of course you use different keys for the different layers. One uses layering not to compete directly with the underlying cipher but to add a fall-back security.

Of course you use different keys for the different layers. One uses layering not to compete directly with the underlying cipher but to add a fall-back security. This immediately demands that one must not reuse the key. Since this is simple to understand and to verify, it is far less important than what you make it seem. Also, you then do not get lower security! You still have the full security of the underlying cipher. Why all the scaremongering? Crypyography is a science and craft just like other sciences and crafts. One can do misstakes with severe consequences, but that is the case in other sciences and crafts, too. If more people understand cryptography, the better. We should encourage playing with it, not telling people not to touch it.