First, make sure you have a solid understanding of the themes and genres of both novels. Then, set up a schedule that allows you to work on them regularly without getting overwhelmed. It can also help to have different writing environments or cues for each novel to keep your mind focused.
It depends. Working on two manga simultaneously can be challenging. A mangaka needs to have a lot of creativity, discipline, and the ability to balance the workload. But some very talented and experienced ones can pull it off.
Yes, it's possible for some very productive and organized authors to do that. They might have a clear plan and the ability to juggle multiple storylines in their minds.
In some cases, it's fine. But it depends on your skills, schedule, and how complex the comics are. You need to make sure you can give each the attention they deserve without sacrificing quality.