Two reasons why this probably happened. 1) you paid too little, and 2) you probably had people in-office too so the remote employees were second class workers.
The solution is simple: pay above market rates and create a truly remote culture.
If they can keep up both of their jobs and the contract allows it, I don't see the problem.
The reality is that a lot of jobs don't require that much time and low performing people can go unnoticed for years.