I can't remember the details, but a couple people have mentioned a program that will allow you to do this
Indeed. The program is called
gates.to and it allows you to fly the same plane as another person, together, during network play on two different computers.
In other words, he can do the taxi whilst you control lights, close the door (he can actually see it on his PC if you open the door!!), pour yourslef a glass of soda (its great to have a copilot

) or however you want to fly together.
Its really strange the first time in FS2004 when you see your plane start to taxi, and takeoff, and even land "by itself" without you doing anything...

You can also choose to be instructor and student, and I think added to that you can have 6+ passengers. Excellent when you have a plane with a cabin and want to give your friends a virtual flight.

Its excellent fun. Ive done loads of flights with gates.to.
Plenty with Antei, for example in the Aerialfoundry Hornet I would take the front seat in the virtual, and he would take the back seat. Its awesome to have someone fly you at mach 1 whilst you have time ot sit back and look out the window..

Ive also done two or so flights with Hans Lemmen. He's a friend of my parents and a real pilot at Sn Brussels on the Airbus A330-300. Those were some of my coolest flights (and most nerve-wrecking, you always want to impress!

)
So if you havent yet, get gates.to. Its so cool, for example if you switch on the smoke system on your pc he will also see the smoke on his, same for doors, lights, flaps, spoilers, and all default and many addon systems.
Unfortunately there isnt support yet for FMCs, obviously that would be a bit complicated.
However many complex planes like the Dreamfleet 727 work exemplary, and it feels sooo good to get through your procedural checklists in a third of the usual time.

Well, just try it.

Heres the site...
http://www.gates.to/LoadPage.php?page=Welcome