Let's start with the basics: a multiple viewpoint novel is one in which two or more members of your cast list are viewpoint characters - that is, those characters through whose eyes we witness the events of the novel and whose thoughts and feelings we have direct access to.
Or to put it even more simply: if different chapters are narrated by different characters - chapter one from John's point of view, chapter two from Helen's - it is a multi viewpoint novel.
Multiple viewpoint novels are common in literature, so it would hardly be a risky choice if you chose to write one yourself. Before you can decide, though, you need to understand...
Is it better to stick with one viewpoint character, or does having two or more characters in the spotlight, as it were, add dimension to your novel? And does writing a multi viewpoint novel make the planning and writing more difficult?
First the "difficulty" issue...
While it is certainly the case that writing from more than one point of view is more complicated, it isn't that much more complicated.
You have to learn when and how to switch viewpoints effectively, but if you follow the advice on how to do this lower down, you won't have a problem.
Using several viewpoints also makes the planning of your novel slightly more complicated, in that the chapters told from each viewpoint effectively constitute a separate subplot. But, again, it really isn't a problem.
A bigger issue than difficulty is that of "focus"...
Put simply, the more viewpoint characters you use in a multiple viewpoint novel, the less time the reader gets to become familiar with each one, which can result in an unsatisfying read.
It's like going to a party and having to move on to talk to somebody different every twenty minutes, just when you were getting a good conversation going.
My best advice, then, is this: stick with just one Viewpoint Character unless you have a good reason to use more.
What constitutes a good reason? The story you are telling, and nothing else.
If you believe that sticking with one viewpoint character is too limiting and that you won't be able to tell the story as well through one pair of eyes as you could through several pairs, write a multiple viewpoint novel.
But even then, keep the number of viewpoint characters to the absolute minimum required to do the job.
Virtually every novel featuring two or more viewpoint characters is written in the third person point of view, and for a very good reason: it is the most logical viewpoint to use.
A Third Person Story, remember, is narrated by that invisible, godlike witness to the novel's events (or the magic camera, if you prefer that analogy) - and it seems perfectly natural for this narrator to choose to slip inside not just one character's skin during the telling of the story, but several.
What seems less natural, at least to me, is for two different first person narrators to sit down and tell their respective stories at the same point in time after the novel's events are over.
So here is my best advice...
The next point to make is that viewpoint characters may or may not receive an equal share of the chapters in which to be in the spotlight, depending on the kind of story you want to tell.
Suppose you want to write a detective novel; here are some of your possible options...
You can see, then, that who you choose to be viewpoint characters, and what proportion of the chapters you allocate to each of them, has a profound effect on the story you are telling.
So if you are at all unsure how to structure a multiple viewpoint novel (or whether you want to write a multiple viewpoint novel at all), it is worthwhile working out all of your possible options, like I did with the imaginary novel above, and going for the version that excites you the most.
Like I said, there is something not quite natural about a first person novel told from more than one viewpoint. To my mind, they would always work much better in the third person - though that could well be due to little more than my own personal tastes, so don't let that put you off writing one.
Technically, you can have as many first person viewpoint characters as you like (a first person "ensemble piece" featuring a dozen narrators/viewpoint characters, for example), but I would advise sticking with two first person viewpoint characters as a maximum (a dual viewpoint novel), or else writing in the third person.
What kind of stories would be suited to the first person dual viewpoint approach?
In short, stories in which the two characters are of more or less equal importance - for example, a love story. Or that idea for a detective novel above in which the detective and the murderer are the novel's equal protagonists, and in which they share the chapters between them.
A practical consideration with first person prose is that it isn't always immediately obvious who is speaking...
You need a practical method for overcoming the problem, and the most obvious way is to label each chapter with the character's name.
So in a first person dual viewpoint novel, for example, Chapter One might be entitled "Fred", Chapter Two "Mary", and so on.
Or you could split the novel into parts, with the first part called "Fred" or perhaps "Fred's Story", and the second part "Mary" or "Mary's Story".
First person multiple viewpoint novels are relatively rare, but one good example I managed to find in my own novel collection is Jane Hamilton's A Map Of The World.
(Interestingly, I also found an example in my DVD collection: Martin Scorcese's Casino - watch it and you'll see what I mean.)
If you have made it this far down the page, you have probably already decided to write a multiple viewpoint novel - probably in the third person but possibly in first person point of view.
The next thing you will want to know is how to switch from one viewpoint character to another without committing the cardinal sin of writers: confusing the reader. I deal with precisely that in How to Switch Viewpoints...