More than 1 WIP split personality disorder
Like some other posters, I'm a 'newer' writer. Newer in the sense of finally putting pen to paper and writing out the novels I've been fleshing out in bits and pieces via notes over the years. Yet I've always had ideas for multiple plot lines going on in my head.
I think this might also partially depend on whether you are working on a novel series or a one-off. I'm actually juggling both at the moment as I work on a fantasy novel series of five (first book nearly complete in draft but for a few segue events with plot notes and some scenes drafted in the later four books), while also hammering out several random one-off novels that popped into my head and would simply not go away until I brought the characters to life. That resulted in one other complete one-off that I'm now in beta reader and editing stages, and another new novel that could have spin-offs.
I find myself having to write and think over scenes throughout my fantasy book series though, as I can't box myself into a corner in the first book by not knowing my end game. And sometimes seeing and hammering out the final character arcs in later books helps me go back and flesh in the details as to how they got there in the beginning. I'm certainly not the most linear individual in my writing style.
Writing to music is one way in which I keep everything straight, as for me, certain songs can evoke the mood or the sense of the characters quite vividly. Often I can listen to the song I was using for a particular scene and swing back into that novel.
I also find working through the one-off WIPs rather refreshing, as it gives me time to noodle on the longer fantasy series in the back of my mind. And it keeps me from rambling in long speeches as I have a wont to do in those novels after having kept things short and sweet in the modern novel WIPs. (Shocker after reading this meandering post, no?)
Of course, I like juggling multiple projects in my other professional and personal pursuits, so I'm not surprised I tend to do this with writing. Go with what works!