You can make a few combinations, depends of money amount. You can even have a full in-car view, but it will cost you. A lot.
First you need a FireWire hub, 6-pinned, with 6 ports. Then cables. Notice that number of cables depends of number of screens you wish to have, but buying 6 will allow you to make a sweet iLink parties at your house when your friends find out you get yourself a hub. Cables should be FireWire cables, 6-4 pinned (6 pinned end goes to an hub, 4-pinned to the PS2).
From that point things are easy, but more expensive. If you wish to have only side-views you'll need 2 extra PS2 consoles, 2 extra copies of the game and two extra TV sets. For full in-car you'll need 5 consoles, 5 extra copies and 5 extra TV's.
Setting up is easy. In Arcade mode there is an iLink option. Before going for an set-up you must determine which of the consoles is "leading" one, AKA, which one has the number "1" when in iLink mode. That console should be attach to an TV in front of you. Once you are ready for set-up, you go inside the iLink with every console. Notice there'll be lots of cables and controllers around you and that you must be very fast in pressing the X buttons

. For the console "1" you must choose the "race" option, and for the other consoles "spectator" option. When the set-up is done, you have to change the views in "spectator" consoles depends of the sides of the car you wish to see. Notice that consoles started in "spectator" view are displaying all the replay cameras, so you can choose between all of them. For an side-views you must choose thee side-views. For an full-cockpit mode (5 extra everything), you choose side views for the side TV's, for 2 consoles on your back you choose the side-view mirror cameras, and for the TV that's on you back you choose the rear-view mirror cam. You'll soon notice that there is no purpose of having all that hardware connected because you should mount a real mirrors to use all that in real life, but you'll have an Ultimate GT experience.
And in some point, there was shown a set-up with 2 extra everything that was allowing a wider first-person perspective (as seen in that lovely Subaru simulators) but I don't know how to come to it. I had an chance to connect full 6-consoles in iLink, but that option never came on. Intresting thing is that I actually saw hardware under the hood of Subaru simulator and it had 4 PS2 consoles, but only 3 screens, which I explaines to myself as "this is not for mortal users". So, try your luck with the "2-extra-everything" or "5-extra-everything" set-ups I described above.