I'm lazy and saying "folder" is fewer syllables than saying "directory".

Are you getting 2 public IP addresses from your ISP? Do you intend to run different LANs for each property? Are you the wifi admin for both properties or do you intend for the tenants of the secondary to bring their own router+AP and hook up to an Ethernet drop from your network? It's kind of hard to make network recommendations without knowing the full context around how/who will be using these networks.

Even if you want them to be completely separate, you still only need 1 router as long as that router has multiple NICs or can vlan to whatever switch your wireless Ethernet bridges are connected to.

I don't trust RAID, been bit too many times. ZFS pools, baby!