Having been in this situation my whole life, going back to being a Machinist, there is one important thing for the younger guys to remember:
If you are learning, keep learning and it will pay off at this job or the next. Perhaps as you learn to your tasks completed better your boss will realize that he does not need to keep the other guy around, that still leaves you where you are at. My point is if they fire the other guy you may still gain nothing except even more work.
If you are gaining knowledge you will be rewarded eventually. As I stated earlier, I began my career as a Machinist, actually just running a saw cutting stock for the lathes. 17 years later I have worked my through CNC Programming, Production Management, R&D Management, Project management, Product Design... Here I am today working in Engineering (without a college degree) I have hands on experience with all aspects of manufacturing (welding, programming, laser machining, quality control, design, analysis...) and I am not the highest paid person in my department. I get up every day and go to work with a smile, do the best I can do, learn as much as I can learn and come home feeling pretty good about myself. 17 years ago I was running a cut off saw in a third rate rats nest of a machine shop making $5.00 an hour, but by taking on everything thrown my way, showing a will to learn, making the effort to learn and achieve, in return I have been rewarded with a good career that many machinists could only dream of. I have had the fortunate opportunity to work some large companies on some very large projects, and I love it. Life is long, look at the big picture, appreciate where your life could be heading.