Simplest way is to replace your USB modem with a Router/modem and connect both PCs to it via ethernet cables. I use and recommend the D-Link 604+ which has 4 ethernet connections and would deliver exactly what you want.
What is ransomware and how do I protect my PC from WannaCry?
What I learned from my mentor, Oscar-winning VFX supervisor Phil Tippett
Siri vs Google Assistant