You can't apply different stylesheets to different parts of a page. You have a few options:
The best is to wrap the different parts of the page in divs with class names:
<div class='part1'>
....
</div>
<div class='part2'>
....
</div>
Then in your part1 CSS, prefix every CSS rule with ".part1 " and in your part2 CSS, prefix every CSS rule with ".part2 ". Note that by using classes, you can use a number of different "part1" or "part2" divs, to flexibly delimit which parts of the page are affected by which CSS rules.