[Bill Douglas]

I did a simpler version of this but my explanation may give you a few ideas as to how you could set yours up.

The fibre optic cable comes into my house and to my router. Then I ran a cat6 cable to the apartment at the back of my house. Then I set up her router as a secondary router which has a different IP address (must have a different IP address) then plugged the cable into my router. The lady in the apartment plugs her computer, printer and TV into the router and she has her own network name and password for her phone to wifi into.

In your situation you could have a third router with another IP address connected in the same way. Security again is by having their own network name and password.