For me it is always okay. Because if I focus solely on one I would eventually face a burnout. I’m not kidding. Since I would hit a block which I cannot get through or end up just losing interest in that novel.
So for me, I have to work on two to three manuscripts simultaneously to keep the momentum. That each chapter is relatively fresh and possibly reveals new twists or a new way of seeing a character. I cannot just work on one single work at a time, I need to at least have two to three. It was one thing which proved to have worked for me, since I’m on the verge of finishing my first novel after seriously writing for nearly two years.
But there are many downsides. I have many times where I needed to look back because I simply could not even remember certain details. But that can remedied by notes, or some ideas for future chapters. I usually do that from keeping myself from forgetting.
I also have a routine time where I would work on each story, sometimes some have more time, others have shorter depending on how much progress I have made or how long that story is. Or how fast I want it complete? It seems that the only way I can write is that I simply set goals for myself and don’t make excuses, and also habit.
To work on two manuscripts is also to divert your attention. It can be difficult, it can be daunting. But for some writers like me it works because it retains my interest. Since every day I am not staring at the same manuscript over and over, I can quickly find a way to begin. Or sometimes I just add a sentence.
Another reason why I also do this is that I would be constantly thinking about each story on a deeper level and I probably wouldn’t discard it once the novel is finished.
Even though progress can be really slow, but I use this method because to me it’s a way that forces me to write and think at the same time. I also encounter a lot less writers block, since I probably wouldn’t just constantly be thinking about how a plot twist would work then realise that it doesn’t. And whatever I come back to the work with, is that I am always a better writer.