I guess better late than never...

So we've got a failover IP, and a nice trick to be able to get the same WAN IP on both our routers. If you do stateless packet filtering, this is actually enough, and your redundant setup is already finished. However, if you do stateful packet filtering, the moment the failover IP moves to the backup router, your connection will be dropped because it doesn't have any knowledge about it in its connection tracking table.

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.