Look at your job description - are you doing things not listed in your responsibilities and duties?
Now - everyone does do some stuff not in their job description from time to time - but if you're really taking enough load of your co-worker to be working "two full-time positions" then talk to your HR and ask them to verify what your job is; ask them to update your job description; and then ask them for a raise commensurate to your additional responsibilities.
Or, ask for another hire to take extra work.
However, it is just likely that you won't get very far with this - a lot of companies are understaffed - that's just the nature of business. If they see two people getting the work of three people done, they won't see the point in hiring another - the work is getting done. In which case, you have to demonstrate that work is not getting done - clearly showing that it is because there is more work than can be handled when you are already flat out.
Finally, if none of this results in less work load or higher pay, start looking elsewhere.