Starting with two niches means testing two sets of messages, offers and delivery assumptions. Unless you already have a working service and enough capacity, that can make it harder to learn which part of the business needs attention.
Begin with one clear audience you can understand and reach. That is a practical limit on the initial experiment, not a promise never to help anyone else.
Compare your starting advantages
For each audience, ask where you have relevant knowledge, existing relationships and evidence that people want help. Consider who already approaches you and whose problems you enjoy working on.
Avoid choosing purely from the size of the market. A large market is not useful if you cannot reach people or explain why your offer fits them.
Count the work honestly
Different audiences may need different examples, prices, onboarding and communication. Even when the underlying coaching principles overlap, the way you explain them can differ substantially.
If you have time to test one offer properly, splitting that time does not create the capacity to test two.
Keep existing clients in perspective
Focusing your marketing does not require abruptly ending suitable existing client relationships. You can choose a clear acquisition focus while honouring the services you already provide.
Be honest about whether the same programme actually fits both groups. Do not promise two distinct experiences while delivering one generic service.
Define when to revisit the second niche
Record the evidence you want first: a clear offer, repeatable acquisition, manageable delivery and enough capacity to run another experiment.
When those conditions are present, assess the second audience as its own test. Keep its results distinguishable so you can see whether the expansion improves the business.
The first objective is a working service for a specific group. Additional opportunities are easier to judge once that foundation exists.
Related guide: How to choose a niche for online fitness coaching.