If you want to specify multiple authors in your YAML Frontmatter, then you are going to want to use YAML's list syntax like you did with categories and tags, like this:
author:
- usman
- someone_else
This will be useful for dynamically injecting author information into each of the posts.
As for allowing multiple people to contribute to the same article, I don't think this has anything to do with Jekyll or what is specified in the Frontmatter. This is an issue of having your Jekyll content hosted in a shared place (such as on GitHub, like many do) where both you and your collaborator can both work on the file. That being said, be aware that you may run into nasty merge conflicts if you work on that same markdown file in parallel.
Update
This is an update based on the OP's edits to the original question.
A simple hacked approach would be to set your author tag like this:
author: Usman and Someone_Else
This doesn't give you much flexibility though. A better solution, which would require you to modify the template you are using, would be to do something like the following:
First, setup your YAML Front Matter so that it can support multiple authors.
authors:
- Usman
- Someone_else
Now, you modify the template to go through the authors specified in the YAML Front Matter.
<p>
{% assign authorCount = page.authors | size %}
{% if authorCount == 0 %}
No author
{% elsif authorCount == 1 %}
{{ page.authors | first }}
{% else %}
{% for author in page.authors %}
{% if forloop.first %}
{{ author }}
{% elsif forloop.last %}
and {{ author }}
{% else %}
, {{ author }}
{% endif %}
{% endfor %}
{% endif %}
</p>
Resulting HTML:
If no authors are specified:
<p>No author</p>
If one author is specified:
<p>Usman</p>
If two authors are specified:
<p>Usman and Someone_Else</p>
If more than two authors are specified:
<p>Usman, Bob, and Someone_Else</p>