If they are both on the internet, just go to multiplayer and host a game with no1 PC, then enter the IP address of the host on the other machines multiplayer screen....... Fly!

It's better to fly on a LAN connection though as you don't involve the internet, it's a direct connection via Networking cards or onboard networking  chips within the computers. A networking cable is also required.

You need to set up a local LAN network, then fly multiplayer on that network via the games multiplayer options.

I would have thought you are already networked via the router, so why can't you just go and fly together right now via multiplayer?

Ok, first choose which PC is to host, the most powerful is usually best.  On this PC hit the start button on taskbar then choose 'Run'

In here type command then press enter. You get a dos window up, here type Ipconfig.  It should list your IP address. Most Lan ones are usually somthing like 192.168.0.*  Write this number down.  Now choose multiplayer, then host a session etc.

On other Pc click on multiplayer and in the IP box at the to ptype in the IP you wrote down and press enter.  The name of the game should appear in the window below.  No highltight the game and hit join!