You cannot buy X5 now so it will have to be RH6 and you will
both need the same if you are working on the same project.
Don't even think about working on a project strored on a
network drive unless the project is under version control. You must
work on topics on your local drive. With version control, the
required topics are checked in and out from the network so that you
work on them locally.
The same applies to the remote worker accessing your hard
drive. It will effectively be the same as a network drive and you
will have problems.
If you will both always work on the same discrete areas, you
could split the project and create merged webhelp but that would
need a lot of links to be changed if you have links between topics.
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