...you BOTH have to have your own registered version of the Sim (either FS2004 or FSX) installed on your individual computers.
....as Andrew says, you can then fly together using the Multiplayer option in the separate Sims, on both computers...
FS 2004 Pilots can fly with FSX Pilots, and vice versa!
...which is what the rest of us do...

...!
You wont upset Uncle Bill then!...

...!
Paul...G-BPLF...FS 2004.... 8-)...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.