how can I archieve the following: I have a linux system behind a gateway. I want to have two IPv6 addresses. For both, the first part should come from the router (as usual). The preferred address for outgoing traffic should be one whose second half is random and changes in fixed intervals (read: Privacy Extensions). The second one should be for things like accessing my computer via ssh and, because of that, have a static second half (and be totally static link-local).
This is the default behavior on most systems. If you enable Privacy Addressing on your computer, you will still keep the MAC-based IP address.