Yes, you can connect two routers to one fiber modem, but understanding the 'how' and 'why' is crucial for optimal network performance. Assume you have house with direct access to an optic fibre cable (FTTP). In the basement, there is the ONT+residental gateway device that converts the light impulses to Ethernet. This ethernet will then go through a 1 Gbit/s switch, and rout two ethernet cables to each floor. On each floor each ethernet cable will be connected to a router, which will then distribute the internet. A common solution is to connect two routers on the same fibre optic line.