There are some options.
The first is that you set up source control and you then
check out whichever files you want to work on. There are topics on
the Adobe site about that. Also there are some links to them on my
site.
After that it gets a bit more tricky. You cannot both work on
the same project at the same time. Yes your colleague can work on a
copy and you can import her work but how will she create links to
topics you add after her copy is created? How will you create links
to her new topics? Then there are index and TOC issues. Remembering
which topics she has added and amended. There will be tears.
What you could do is consider merged webhelp (or HTML help).
You would have to split the project into separate content areas
that you both work on. You will have two outputs but to the user
they will appear as one.
There's a topic on my site about merged webhelp.
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