So, you've got not one, but TWO ideas for a book and you can't decide which one to work on first. I know that feeling, I've been there. I worked on one and told myself I could NOT do two at once. It was an insane idea and it would just lead to tears. Do you know what happened? I couldn't get past the 11th chapter of the one I was working on. I was completely blocked and so taking my own advice in regards to writer's block (a piece can be found here) and I put it to one side and started writing the other book I was so desperate to start. That's when the trouble started.
You see, I got to the same point in the second book as I had in the first book. Only, I now had a load of ideas for the first book and wanted to finish it. I also had a load of ideas for the second book and didn't want to risk losing my creativity flow as it were. So I made, what some people would call an insane decision, and I decided to write both. I would work on two chapters from book one and then swap over and do two chapters of book two. And you know what? It worked!
So, when I finished those books and was happy with them, I decided that to hell with it being insane, this worked for me so I took on another two books to write at the same time. I've stuck with that practice ever since. I'm actually on the verge of finishing books nine and ten, and already I am planning the next two books I will write. It means for me that I get one book from my series written, but I also get a standalone book done too. It doubles my productivity. Or at least it does when I'm not blocked.
It's not easy though and I do have some ground rules that I try to stick to at all times. I mean, sometimes I don't, but that's why I said try! Here they are:
Rule Number One: NEVER write two series books at the same time. Think about it, if I was to write book four in the Dying Thoughts series at the same time as book five, I would mess up the continuity. I would have to stifle what I put into the books to make sure that the time lines matched up and that would limit my options on where I could take Tara and Kaolin. It would also meet writing ONLY Tara and Kaolin and as I mentioned in my "writing a series" post (found here) sometimes that's just not a good idea in general. Some people can do it, but I'm not one of them who excels at it. So that's rule number one.
Rule Number Two: Stick to the plan. I don't mean the book plan, I mean the plan of how many chapters of which I'll write before switching. I know that there are some exceptions to this rule. One is that if you're writing away and feel like you can go further than x chapters then you should. In regards to this rule, I'm talking about when you're blocked after one of your chapters and you think, "I'll just swap over and come back to it another time" - if that works for you, great. It does NOT for me. It does mean that sometimes I will spend longer on one book than another, but it also means that they are generally at the same points in the story. This helps me keep myself grounded in the right plot for the right book. Again, this is a rule that has some wiggle room in it.
Rule Number Three: ALWAYS familiarise yourself again before starting the next chapters. I know that this seems like a stupid rule, but it's a simple mistake that people can make. If you've just spent a few hours/days/weeks in universe x, and hop over to universe y with only a vague recollection of what just happened, then you need to familiarise yourself again. If you don't, this can lead to a headache when it comes to the editing process. It can be a nightmare if you've finished your first draft and you send it to your beta or editor and they read it and think you must have inhaled some crack because none of it makes sense. It's always better to have re-read the previous chapter(s) to make sure you're brain is firmly in the right universe before you start adding words.
Rule Number Four: Writers block will happen. Work through it. This rule generally goes back to rule two in that some people will find when writing two books that they get writer's block and then move onto the other book and it flows better. That doesn't really work for me. Usually if I have writer's block for one, I have it for the other too. So, some wiggle room for sure. As a writer, (or anyone expressing themselves creatively) then you'll know that getting blocked happens, it's what you do to move through it that helps determine when you get through it.
Rule Number Five: ENJOY YOURSELF! If the thought of writing two books at once scares you to death, or brings you out in a cold sweat then don't do it. It's not for everyone. Some people find it helps, others just hate the whole idea. For me, it works. Like I said, it keeps me productive and keeps those elusive muses singing. For you, it might be a nightmare waiting to happen and if that's the case, don't put yourself through it.
Those are my general rules. I have other minor ones, but those are the major five that I stick to. People ask me, "don't you get confused?" and the answer is no, never, I have never gone to write my book in universe x and mixed up the plot with universe y, but that's just me. I plan a lot (see here) and I'm slightly obsessive about how much I plan. I do what I do because it works for me and I know of other writers who feel the same way.
The best thing is that I get to play in two sandboxes at once and that is, to me, one of the great things about writing books. You get to create and explore a universe so different from your own and it's great. Now, if you'll excuse me I have some chapters of book two to write so I can finish book one next week!