Must meta-analyses or systematic reviews have more than one author? 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?
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.