Used before reset, now looking for proper dual-lan config.
Dual LAN or dual WAN? For proper dual WAN as primary and backup (i.e. no load distribution),
this article has it all and doesn't require routing marks to work.
For dual LAN I don't know what you have in mind.
If using routing marks, the thing is that some sources say that if no route is found in the explicitly indicated routing table, the routing table main is used as a backup. I haven't tested this deeply, but I'm afraid that a default route is considered valid also for local interfaces. So if you need to use named routing tables, local routes must exist in all of them, otherwise the route-marked packets for another local subnet are sent out via the default route matching that routing mark. The other (and easier to understand three months later) possibility is not to mark packets with local subnets as destinations of course.