I run a bitcoind that is available on two external IPs at the same time. You need to forward your port 8333 on both external connections on the router to the machine running bitcoind in your internal network. Plus you need some rule/process for determining which outgoing connections go out via which interface.
If both connections are alive at the same time you should appear to the world as to separate nodes on your two external ips. If you only turn on the secondary ip when the first fails you would have some time for the existing nodes to go stale. I think it's 90mins. Of course your outbound connections should refresh sooner but old connections may sit there for 90mins before formally closed.
Also from the point of view of bitcoin your external ip has changed. You need to probably implement this PR for it to work well, otherwise it may never advertise the new external ip:
https://github.com/bitcoin/bitcoin/pull/5161Also see
https://github.com/bitcoin/bitcoin/issues/48In my case I do not need ot advertise the second ip so it's not an issue.