Is it possible to play with 2 players, instead of 4

Ok, it's a little bit tricky but possible in normal mode. I think it's nearly impossible in higher difficulty modes because there are simply too many civilians walking around the alarm buttons, and you can't keep an eye on the three at the same time (especially on the bitchy one nearby the staircase).

To do so, after you took care of the video cameras, immediately jump on the welcome desk and spam your yelling key (default: F) to get everyone down. Also aim at the AI and spam again to force them to get closer. Try to get at least one of them behind the desk. Use this method when you have to walk away to take care of the guard, of the maintenance guy or to set up the fake sentries.
You'll have to time it well; you want the AI to stay behind the desk while you are away. So call them and then run where you have to go; you'll have something like 10 or 20 seconds before they reach and then leave the position you gave them, trying to rally you, and therefore giving the opportunity to civilians to push the alarm button.

After this sneaky sequence, you won't need crowd control anymore so it should be okay.

To the guys I'm playing with and I, the only really revelant lines are those that are given during the second doctor's turn. Listen to

1. the blood pressure (sometimes helping, sometimes not) but mostly
2. when the patient will be able to come out of the hospital (always a right indication so far).

You'll then have two main cases:

• If the patient's sickness has been identified/diagnosed and his treatment is going well, he'll be able to go back home soon, meaning it's not the guy your are looking for.
• If the doctor says she's not 100% sure what he suffers from and/or the guy has to stay for further examination and treatment, this is your guy.

To complete my previous answer, while playing multiplayer, different players will hear different things from the doctor; the estimate time before leaving the hospital is the only one that will point in the same direction for everyone (even if the information can be worded differently from one player to another).