Multiple system hook ups(w/ PS2)

I think this is what you are looking for. It allows you to hook up 5 systems at once. My friend has one and it really makes things a lot less confusing with not having as many wires...

It's possible depending on what kind of tv you have. If your tv has more than 2 sets of AV jacks then yes you can. Just put your cords into each seperate AV slots. That is assuming your using the standard jacks that come with your GC and PS2. If your using coax cable then no, most tv's only come with on coax hook-up (place where you put your cable tv cord).
There is also a peripheral that you can buy from Pelican that allows you to have 4 or 5 seperate consoles running at the same time. All you do is plug all of them into the device because it has 5 seperate AV ports and then press the button for which ever console you want to use.

Here is the Pelican RF Switch and here is the MadCatz RF Switch in case you don't want to look around...