Two Xboxes, two copies of Saint's Row 2, two televisions (or a moniter? I think you have have your 360 display on a moniter screen), and an ethernet cable (Like the one used for Xbox Live) plugged between the two, or being linked together in the same network or some crap.
So if you're looking to play co-op with a friend of yours, it'll be an expensive hassle which is the unfortunate part of system linking.
Currently Playing: Saints Row 2, Fable II Gamertag: ShadownetNinjaX
^Plus the screens would be all squished and look like crap. Probably not worth it. And also everyone would whine about how bad the co-op was and the game would look worse when it was only trying to look better.