It is always a problem, that a new project is started, when the first one isn´t even nearly as finished. You are working on two things, but cannot concentrate your ideas on one.
Around 80% of all Devs, who started a second game when the first one wasn´t done, never finished any game. Because they get bored or loose the idea for the original game, which was maybe succesful but the second or third isn´t.
But its the same problem as in a real company. But there you have more pressure. You have to finish your given project before starting the next or you get fired. Deadline is the word of wonders at this point. You need a schedule and a plan on how and when you finish which part of the project. Long time i worked as a Project Manager with a Team of 20 people and there was not a single day, at which you don´t look at the date of deadline.
But in games its a little bit different. Sadly. Most devs just don´t care when their game gets finished or even have a plan. And either they get bored of their original project or don´t have any more creative ideas on how to push forward.