Well, what your ISP is saying is "Don't share your internet connection, we want to sell you one each".
2 Media converters for fibre seems odd - the topology most fibre ISPs (in singapore) use is an ONT (which will take one fibre input, and will let you use one of its ports to connect to your gear) to a consumer router to everything else. Even if it was a simple converter, two media converters means two ethernet connections in, with a maximum of 2GBPS and the need for fancy things like bonding. Its strange.
I'd note, laying lan cable between houses is tricky. You'd want to have it somewhere where it won't get taken out by environmental hazards, wild backhoes, rabbits and so on. You'd want conduit and suitably rated cable in many cases. You'd need to crimp the cable and so on.
I'd doublecheck why he needs two, and see how pricing differs if its a single home that has it.