hi bernd, as as i know it is not possible. i have the same issue before then i change to flat network where no floating ip define but still the same issue. anyway, from there i can see that dhcp will only allocate one ip for the interface, then you need to manually force the instance to use the fix ip to you local interface.you can do it by telling your instance using below command:

ip addr add <ip>/<mask lenght=""> dev eth<number>

you can set it in your rc.local also which will survive reboot.

but take note that you will have to define the ip from the same port-id as it will use the same virtual mac address. if not, your instance will think it is a different interface causing it create another interface and this will not allow you to communicate to that interface event you have bind floating-ip to it.

I think that you can add the secondary ip to allowed_address_pairs proprety of neutron port. I made this with pacemaker vip