Ok I’ve learned a couple things and there are some boneheaded things in this last post:

First:  the FEC0:: trick is deprecated.  There really is no reason the FE80:: addresses should not work across a switch, so long as there is no router involved.   It might be an OS option.  I’ll have to check.

Second, the address is fe80::x:x:x:x/64.  The top half under the fe80 is all zeros.  That is the netmask, not the top half of the MAC address.  So, while it is cool that they have the same top halves, that is not why the two mac addresses are on the same network.