What is the best practice when developing such a SaaS? - Two separate apps for security purposes - Single app where features and UI are based on user's credentials
PS: The database would be shared in case I developed two different apps
Find someone willing to pay for it before it is built because that means you are solving a problem someone is willing to pay for. Finding a real problem to solve is often harder than imagining a problem to solve.
Paying customers is the important problem. Security only matters once that is the case.
Good luck.