Finding a voice, finding another one

Sometimes you need more than one voice to tell a story. Sometimes it's necessary to slip points of view, to show what another character's experience of the story is. But getting your reader to make the change with you, to put up with parallel storylines offers challenges - if one voice is very strong and compelling they might not want to leave it. I'm sure I'm not the only person who skipped all the Browning-esque poetry in A.S.Byatt's Possession (ooh, how naughty). I quite like stories with fairy tales in them. I had this funny old book when I was a kid called R my Name is Rosie about a girl who lived in a hotel run by her busy mother who really wanted a dog. She and the bartended took it in turns telling each other this story about a princess called Rosalind (I might be misremembering some of the details). Anyway, there was this floral design in the margin to indicate where it slipped into the fairytale, which meant sometimes on rereading I'd skip one of the narratives and just read the other, which was very postmodern of me. I loved it though, I read it heaps as a kid. Apparently Lili does something similar in Scatterheart (which seems to sell out rather quickly in Readings, I must order in a copy).
The Indigo Girls which comes out next year is written from two first person points of view - I wanted to show the interior of the friendship of two very different girls from both perspectives.
The novel I'm writing at the moment has a very particular voice. I feel like I might need to show things from another side, but I am not sure I can do it. I've been looking for a second voice but the voice is elusive.
For me finding the voice is the difference between being able to write a novel and not. It's multi-stranded. First person or third (for me, pretty much never second - hmm, maybe I should try it now)? What's the vocabulary? What's the dialect? If it's first person is it an interior narrative, is there an implied audience, how unreliable are they? If it's third does it attach itself to one perspective or move between characters? If it's first have I chosen the best person to tell the story from? What if it's from this person's point of view, or this one's? The voice for Undine came to me in snippets, lying in bed or walking the streets. I wrote quite a lot of it in my head before I wrote any on the page. It's third person and slips p.o.v st first just between Trout and Undine and then one or two other characters as the trilogy progresses. The Chomp I wrote for Penguin, Josie and the Michael Street Kids I actually wrote in third person and converted into first at some point in the writing process (which is making writing the sequel, something I pick at when I'm stuck or bored of everything else, interesting - should I use the same technique?) Indigo Girls was always first person, though it took me a while to get the two voices right, wanting them to be distinctive but not overly jarring, to keep the reader pulled through the story without them having to readjust with every new chapter.
The first person voice of the novel I'm writing now (tentatively titled Only Ever Always) came easily, sometimes the voice is what gives you the character and that was the case here, I got character, setting and atmosphere all from writing a few paragraphs in this voice. Which makes me wonder if the trickiness of finding a second voice is a resistance of a different kind, perhaps it's my way of telling myself not to separate the strands of the narrative at this stage. And yet I'm not sure I can write a novel in bits and then stitch it together, like a quilt. Does anyone else write like this? Every novel I've written I've written from beginning to end, in one document. Of course there's some cut and pasting and shuffling about at the end, some adding and subtracting. How do you put in the stitches at the end, how do you make it hang together?