Amazon customers connect their Facebook friends to find people with wishlists. Someone who has the contact could have information that could link the two accounts, for instance if the OP has someone in their friends' wishlists and they have someone with OP's name in theirs. Or could be connected to a subsidiary like IMDb.
It's probably not cookie-based, or not only cookie-based. Browser fingerprinting and user profiling have gotten very sophisticated, and don't think the big 4.5 aren't using these techniques extensively.
They take the number of people from the same IP into account. IPs are broken down into public IPs vs private IPs based on traffic/timing of usage etc. There are research papers on this sort of feature contruction using only IPs. Cross device especially uses it extensively to be able to probabilistically ascertain if the person from your house who is checking their phone is the same person who checked smth on the Desktop computer last night based on your online timing, IPs, behaviour over the day. They can figure out, for instance, your office vs home browsing timing, interests etc with the same methods.
From the same network ? If both browsers are from different computers (worse still from same computer) from same network then the externally visible ip address is the same isn't it ?