Ok well it depends which game series you want to purchace. Splinter Cell is generaly sneaking about and stuff.
In answer to your question you can play 2 player co-op on one machine for most 2 CL games such as RB6:Vegas.
Rainbow Six:Vegas
Is really good. The Story Line takes you through Casino's and its not run and gun because if you get shot you know about it, you can only take a couple of shots before you will be downed so it requires team tactics and good use of the cover scheme.
The multiplayer is fantastic with plenty of modes for you, ones ranging from Team work based to indivudal, there is Attack & Deffend where the assult team has to retrive a package, there are a bomb modes,VIP assassination, lots. This game is fairly realistic so it make you more carefull whilst playing.
You can order it cheaply here:
Play.Com RB6:VEGAS
Ghost Reacon
I haven't played much of these but from what I understand this is set more in the outdoors and gives you a futuristic HUD, with 'picture in picture' so you can see what your team mates see.
It is easier to identify enemeys because they get outlined as hostile due to your futureistic HUD. This is squad based warfare mainly. I find the story line more fun and less frustrating than Rainbow Six Vegas,how ever less re-warding.
The Online gameplay is fun, it takes a bit of getting used to in my oppinion but I don't rate it as good as RB6.
Here is where you can buy
GRAW
And here is GRAW 2:
Play.Com GRAW 2cheap!
Remember if you go to market place you can always download demos, there is a demo for RB6:Vegas multiplayer and there are demo's for multiplayer & story for GRAW 2 (Ghost Reacon Advanced Warfighter 2)