I don’t know of anything more difficult about novel-writing than pulling off multiple story arcs. I don’t mean a main plot plus a subplot or three, which can be a little tricky – I mean when you have an ensemble cast of major characters, for all of whom you have to shape a transformative change worthy of a novel-length storyline.
Personally, this is one thing I can’t do without an outline – at least a rough one – to guide me and help me keep track of the big picture. It’s easier to keep everything in mind with a concise reference to help solidify it. Plus, it gives me an excuse to color-code everything, which means I get to use markers or colored pencils, which is always a plus, IMHO.
In a way, it’s easiest if I think of it as a map of the book, rather than an outline, and I have to plan a route for each character to reach the destination of the climax, whether that’s the same place for two or more of them, or totally separate towns. Then it’s just a matter of travel planning so I know what they’ll encounter on their journey. Physical battles? Confrontation with someone they thought was an ally? An abrupt and ugly revelation about him/her self? What do I want each person’s story to be about? How can I bring them to their finest moment within that? How can I bring them to their worst impasse or their ultimate failure within it? And of course, where do the different characters’ paths cross? Do they trip each other up, or spur each other on? In what ways, and why?