Just like doing two jobs at a time? It won't work always, there is one of them have to let go while keeping the other one. Now, if we consider ourselves to become a full-time trader then we have to focus on it and forgot the other as trading doesn't just work for an hour but hours. if we can spend more in the market, that was more important and this trading actually requires ample time to analyze the market trend so well, or else, we have no reason why we wanted to become a full-time trader.
It is not that full-time traders can make profits. Full-time traders spend a lot of time in trading every day. They are more susceptible to market sentiment and even greedy psychology that leads to trading failure.
Although part-time traders have short trading hours, if they can analyze correctly and have a trading strategy, they can also trade successfully and make good profits.