I don't mean to start a shit storm, but the term 'multi-task' is one of the most abused concepts in modern day business/life. It is a myth that we can do multiple tasks as effectively as if we were focusing on one task at a time. What is true in physics is true in life, there is no free lunch.
If you can do task X at a 95% level and task Y at a 95% level, then doing both tasks simultaneously will result in completing them at something like 70%. Now if 70% is fine, then sure, have at it. But if you think you can do both tasks simultaneously and still get that 95% level for each, then you are dreaming.
Similarly, we must distinguish between being able to do something versus a preferred method.
Can I chop a tree down with a sledge hammer? Sure. Does that prove its the preferred method? No.
Can you write two books simultaneously? Sure. Does that prove it is the preferred method? No.
IMHO, writing two books simultaneously isn't that hard. Writing two books of publishable quality simultaneously is the kicker.
I personally am a fan of what CeCe said. Namely, writing notes/outlines of that 'other' great idea so you don't lose anything and can come back to it later when you can give it the attention it deserves.
Yes, I know we like to say "What works for you." But in this regard, I will have to throw the flag on that. Write one book at a time. Or put another way. One book should be the 'focus' and the primary while the other book can be on the back burner.
So if you have to split your energy, do something like 90% - 95% on Book A and 5% - 10% on Book B.
Why am I being such a hardass?
Well, it is my belief that starting a book and writing a book is fairly easy. The challenge is FINISHING a book and making it your best. Trimming the fat, finding the write words, REWRITING and EDITING until looking at it gives you a headache.
It is my belief that you need a certain 'focus' and momentum to get you through the entire process. And if you are spliting your energy you may not have enough of that momentum to finish. And before you know it, you end up with a bunch of half finished books.
Anyways, that is my take on all this. Of course, it is just a random opinion, take or leave as you see fit.
Mel...