I am about to start CoCT with only 2 players (and an NPC or two)...
...what classes should I recommend the 2 PCs take?
What would be useful to the campaign?
Off the top of my head, I'd recommend bard, inquisitor, cleric or paladin. With only 2 characters, it will be wise not to fight your way through all the early encounters, so social skills will be paramount. Hopefully you can pick up some allies later, when more fighting power (and healing) is needed.
If I had only two players and I wanted them to have the maximum amount of fun possible I would recommend one of the following classes. I have listed them in no particular order, and I look far more into how the PC's interact with the plot and experiences presented than any "optimum build" rubbage.
1) Holy Warrior (paladin, cleric, what ever) is a pretty good choice, and puts the end game more in the hands of one of the two PC's than an NPC.
2) Probably never been a better AP to play a Shoanti PC.
3) Classic swashbuckling, rapier wielding like PC will find great experiences as well.
4) If you are allowing APG classes Cavalier makes a wonderful choice with lots of great story line opportunites, as does an Inquisitor and Oracle. I just finished tweaking all of the chapters to support APG play and made sure there was one NPC for each of the classes, save Eidelon (which is a great PC choice for an Acadame player). Players in my game are prohibited from APG classes, skills, and feats until they are intoduced through the story.
Anyways, just my 10 cents have fun!