*You can represent more than one company on your web site (and increase your revenue) easily by signing up for an Internet affiliate program that offers multiple merchants.
The offline world works the same way. For example, sales representatives typically represent multiple manufacturers since they need to be able to provide their customers with the right products for each job.
There are some limits to what sets of affiliates your site can represent simultaneously. Internet affiliate programs will each have their own rules about accepting competitive ads. For example, you typically would not represent two strong competitors. Therefore, if you are signing up directly with merchants, you should read each agreement to make sure you understand what is and is not allowed by that agreement.