I wondered why two, and now I know.
344525
I lost the original cap by putting somewhere "safe" while I was charging the band and then forgetting where said "safe" place was.

I found these and was annoyed that they were forcing me to buy two, but there was no other option, so I did.  Now I have the replacements, and I understand why I need two.  It's because these are somehow different from the original, and they fall off pretty regularly.  Usually, I find the one I lost after putting the second one on.  I'm still annoyed, but at least I have a cap.

The color is not different (or maybe not that different) on the ones I received (unlike the first reviewer's experience).
2216402
I watched it twice thinking I was missing something.  I wasn't.
2191727
reason I give two it is because it was open that it is why it is first time I revive an product open
4455615
I thought I was getting the second addition, but was happy with price and info.
3847112
I read two pages and that's more than enough.  I'm passing on this one.  I think I know where it is headed and I choose to not go there.
3576677
It helps to have two in the since one can always be found.
4245660
I was confused at times but I guess that was intentional. Things straightened out in the end. A good read.
3130014
This book helped me understand a series of conflicts of which I was lacking knowledge of. I feel less ignorant to the subject now.
2559348
Have two. When I buy two of something, I must really like it.
1310157
I bought two pairs,  one for me,  the other  for my girl friend, and we both like them.
2013606
I enjoyed the fact that they told two stories that coincide with each-other and shared history of war and the good that comes out of hate.
3916101
This was recommended by a dear friend and I have read it TWICE.
125564
I have to admit, I was wondering how this would overshadow how great the first was, and I was pleasantly surprised by the answer.
Whereas the first one's mystery started with a snapshot of somethign that should not have been seen at all, this one starts with a botched assassination and a cover-up. Also as with the last one, I wondered how this wasn't controversial back then with how gritty it was.
Examples: dirty cops fabricating dirty evidence for good cops; Satanic cover-ups, immense conspiracy dealing with New Agers; powerful demons possessing young children; explicit fights between angels and demons; division within churches; and a whole lot more. The conspiracy in the last one was mind-boggling, this one had essentially melted my mind with how it dealt with a woman who couldn't remember (or rather did not want to) her past and how it dealt with why she was abotu to be killed, and how she has to come to her grips with what reality is about, and when she tells her former employees (satanists and new agers) who she really is, using forward wording that surprised me, let alone the enemies in the book, I was confounded with how forward Peretti is. When the story ended and a final revelation happened, it simply dropped my jaw to the floor and I wasn't sure I wanted to pick it up, for fear it'd keep dropping at the thought of it all.
3532519
I have read The Compound Effect twice and each time I learn things that I somehow missed the first time.
1142085
It was interesting to know the things I think make a difference. I've noticed it in the past but now I know why.
2237489
I liked the first one in my home well enough, that I bought a second one for work.
2021450
Foremost, I now have a much deeper understanding of how much my fore bearers suffered and the low probability of my own existence.

Secondly, I better understand the origins and evolution of anti-semitism which previously baffled me.

I'm looking forward to volume II which I have preordered.