Multiple contact (sub)types for an individual
As long as I know it's a good practice to use multiple subtypes in the same contact.

Doing that you can, for example, create different custom fields for each subtype. So your coaches who are also trainers will have the fields for being coaches and in addition the fields for being trainers.

And if in the future, one of your coaches became a traier, you can add him the subtype and automatically will appear the custom fields for it.

So I will recommend it. :)