If you are taking about the same set of RH project files then
whilst it is possible to have two or more authors working on it I
would not recommend it for your sanity. If you do go down this line
you'd have to be very sure that you would not be overwriting what
the other is doing. Technically all you'd have to do is copy the
source files onto the two PCs and then import the HTM files once
they have finished working on them. You'd have to have one of you
controlling the TOC, Index, etc. though.
A better way of working is to have you both working on
separate RH projects that you can later merge together into another
RH project. For example, you work on project B, the contractor
works on project C and you merge project B and C into project A.
There is quite a good explanation of merged help files in the RH
help.
The final way of doing this is by using some sort of source
control application. This would have both you and the contractor
checking out the files you need to work on. Whilst you have them
checked out, no one else can work on them.