You could test the contents of the cart, for example to turn on FedEx when 1 or more from categories_id 27 is in the cart:
Code:
// global cart if not already done in the function of the shipping module
global $cart;
$chk_freight = $_SESSION['cart']->in_cart_check('master_categories_id','27')
if ($chk_freight >= 1) {
$this->enabled = true;
} else {
$this->enabled = false;
}
where 27 is the Freight only category ...
Then, based on the results you can control whether the FedEx and UPS are enabled or disabled by controlling the $this->enabled in the modules ...