Yes with coax based technology you and all your neighbors are connected in a tree like topology. All your neighbors can indeed technically see your packets. Those packets however are encrypted, if memory serves DOCSIS uses AES.
Yes obviously you could technically double the speed with two cables. You’d also double the cost of cabling deployment. It’s a lot more cost efficient to make advences in the modulation. For example DOCSIS earlier DOCSIS revisions used 256-QAM, while DOCSIS 3.1 support 4096-QAM.
Coax physical infrastructure lasts decades, but we’re able to make new advancements with he modulation every few years.
Perturbator - TERROR 404