It’s not uncommon for players to have both
The agent handles contract negotiations and securing business deals, the manager handles the other stuff. If you want to sign the athlete to an endorsement deal, you negotiate with the agent. If you want them to do an interview on your podcast, you work with the manager.
If it’s related to contracts, endorsements, licensing - Agent, and agents are certified
If it’s related to events, day to day itinerary, interviews, etc. - Managers, who aren’t certified
Now there are some agents who basically take on both roles for their clients, but there’s others who don’t, especially when they have a bigger roster. I.e. Rich Paul isn’t playing both roles for all his clients