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My colleagues argue that a meta-analysis / systematic review needs at least two data collectors (or authors according to them) as a "must". I have not found any reference in the PRISMA (or any other sources about meta-analyses/systematic reviews) that confirms this claim.
Do you know any such protocol or consensus?
From the perspective of an Epidemiologist, with some published meta-analysis experience:
Must is a very strong word. Some people have pointed to where, for example, the Cochrane Collaboration requires it, and PRISMA might not, but generally speaking I've never really encountered a situation where having a single author on a meta-analysis was a substantial barrier to publishing a review.
On the other hand, I would say that a meta-analysis or systematic review should have a second person on the study team. Rarely is the literature being reviewed so clear, so well-laid out and so utterly free of ambiguity that a single person can read, digest and abstract the literature without making any judgement calls. Without having any papers where they search and simply cannot find what they're looking for. Without hitting that one paper they simply cannot make heads or tails of. There should be someone there to double-check your work, or a sample of it at least, to make sure what you described as the system in your paper and what you actually did match up. To look over those papers you've set aside in the "Problem" pile to see if they can see things you don't.
I leave whether or not that person should be an author as an exercise to the reader.
I know some colleagues who essentially begin all reviews with a parallel, blinded double-abstraction of the papers once they've been found - or even begin all the way at the search being carried out twice. I don't know that I'd go that far, but it is extremely helpful to have someone to double-check your decision making against. Despite being careful, reading closely and reviewing my own decisions, I have yet to work on a meta-analysis where I haven't been glad to have a second reviewer (or to be said reviewer).
There is no such requirement. It is frequently helpful to have multiple authors to help sort through the vast amounts of literature being covered, but is by no means a necessity. For instance, a colleague of mine recently published a fairly substantial review article on his own.
However, in medical research, a meta-review serves a rather different purpose than the usual review article in other fields: it is collecting the results of a bunch of different experimental studies, and trying to reach an overarching medical recommendation. That's a rather different research function than a review article in other fields. In medicine, you would want to have multiple people reviewing the data to make sure that it's not one person unilaterally deciding everything independently.
Answer: Although PRISMA does not require such a protocol, Cochrane Handbook does. However, it too does not "necessitate" it, but encourages it (at least to my understanding).
Check out the Chapter 7 of the Part 2 of this online Handbook: http://handbook.cochrane.org/
