http://www.advancedfictionwriting.com/blog/2012/05/23/merging-storylines-in-writ...

Here's a bit to read, but I'm not sure how helpful I'm finding it. I do like the part about not jerking the reader around a lot. Maybe 4 pages works, but you could use more, especially in the background phases.

I would map out this story you want to tell. Look for overlapping pieces that will drag each character into contact with specific things. If it was already a trilogy, then you know which characters you would use and where the pieces fit. You can make this work.