In part 1 I went over the basics of failover. The problem with such a setup is that TCP connections will not survive a failover from ar0 to ar1, or vice versa. The main issue is that both routers have a different WAN IP, and in this case, seamless failover will never work.

Many corporate networks implement router redundancy for obvious reasons. Most of them are probably using very expensive hardware with proprietary protocols, but in fact it is possible to build such a setup for less than EUR100 and use it at home. Just get a couple of cheap routers that are well supported by LEDE/OpenWrt.