These RFD900X modems are originally designed for drones (telemetry) although since they basically are a serial bridge you can pipe anything through them.

You'll need two rfd 900x modems and two separate USB cables (or just buy the kit)
you'll also need need picocom (comes) with most debian and raspberry pi repos
once both modems are connected to separate linux machines the light should turn solid green on both modems.

we're now ready to connect to the modems with picocom over the usb cable.

the command i used for the connection was: sudo picocom --baud 57600 /dev/ttyUSB0

* you also need to local echo on both modems/machines using: ctrl+c+a

you may need to change this to suit your needs.

now anything you type into the terminal will also be displayed on the opposite modem's terminal :)