How do I choose controllers? and how do I use more than two controllers on a web page?

[eluser]Unknown[/eluser]
Hi!

I am a little new to CodeIgniter and Php. I want to build an app and am wondering how to decide my choice of controllers? I have already designed my database and I have my forms sketched already. I have a problem with controllers as per design and usage.

Do I use just one controller per web page? Or can I use more than one controller per page?
If I can use more than one controller, is it impossible to call them independently of each other without having a full page reload? Is that possible using Ajax? Can you point me to a good tutorial?

Thank you very much.