I cannot see a reason to have 2 User collections. As suggested above, you can use “True/False” properties to differentiate a “type” of user. Other ways to do this is to create a relationship between a second collection based on the user collection.
What type of relationship should we use? multiple-one, one-multiple, many-many?
May be you can explain what would you like to achieve?
Because for some cases, like, for instance, clients & coaches, True/False property could be the best option.
And for other cases, like clients & businesses, it is better to have a True/False property AND a separate collection for Businesses (as one user can have several businesses).