start several display on one machine

[Philippe Rousselot <linux rousselot org>]

Hi,

If I want to have several displays running at once on a machine,
I type as su gdm on a command line and I can switch from one displqy to 
another using  ctrl-alt F7/F8

I am using mandrake 8.2.

What files do I need to modify to get automatically two displays?

Thanks in advance

Philippe