Out-of-band communication between two PCs
If you REALLY don't want to use TCP/IP and you want an off-the-shelf solution, then a USB-to-USB null modem cable will work. This is effectively two USB-to-serial adapters with Rx and Tx crossed in the middle. This page has one option that you might like to try - there are likely many others available, and you could make your own from 2 USB-to-serial adapters as described.

That said, I would generally always choose TCP/IP over USB serial for convenience, unless there was some extraordinary reason not to.