If I've created a custom network on my primary unraid server, how can I access containers on that network from a 2nd unraid server, or even my normal network?

 

You need to set up your router to go from one network to another.

That was such an obvious answer....  😣

I'm assuming it would be a good idea to ensure that if I'm allowing these custom networks to route, that I should not have a duplicate network between the two unraid servers?

 

Duplicate networks will make your router very confused. Use unique networks.