Talk to your boss, don't rant about "sexiest projects and reputation" but highlight facts and business values:
- The company will spread this knowledge between two, instead of just one, employees. Even if your colleague won't ever leave the company, the bus factor is currently a significant businesses risk.
- You need those kind of projects to diversify your skill-set and grow. Be prepared to answer which skills you'll gain (and possibly how this will benefit the company).
- If you will ever need any guidance then you'll be able to ask the other colleague. Be prepared to highlight possible blockers you may face, to know what we can't do is as much important as to know what we can. This is important because you need to learn while your colleague is still around and available.
Then stay in silence and listen what your boss has to say (and how he does it). There might be factors you ignore, skills (even soft skills) you need to learn, listen and be patient. Personally I'd try to do not make this confrontational, you're working as a team for the same company then you must work together because you actually have the same goal.
It's just important to "break the loop" a few times and then you'll be a viable choice for the future (don't expect to get the sexiest ones if your knowledge is comparable, his 20 years experience has a significant weight).