second index file

Not too sure what you are wanting to do but the Codex Article Stepping_Into_Templates discusses Template Files Within Template Files.

And for messing about with different themes, just grab ryan’s themeswitcher: see Plugins

Actually, i want to create a new index that uses a different CSS than the current one.

Each theme can have it’s own style.css (see CSS) so as vkayrl suggested, the themeswitcher plugin could work for you.

Okie thanx guys