Hello Joe,
To do the initial calculation of Units, do the following:

Add the 10-day duration task.
Show the Task form in the lower pane (Window > Split) and set the task type as Fixed Duration. Click OK in the Task Form

using the task form, pick the first resource and key 10 hours of work, pick the second resource and key 10 hours of work in the work field. click OK and Project will calculate 13% assignment unit for each resource.

To your second point, you can't do both without a change.

In Fixed duration tasks - if you change Work, Project will change Units.
In Fixed Work tasks -- if you change duration, Project will change units.

in Fixed Duration tasks -- if you change Duration, Project will change Work.
In Fixed Work tasks -- if you change Work, Project will change duration.

So, the moral. Figure out which variable you are going to change, and which variable you want to stay the same -- and set the task type BEFORE making the change.

I hope this helps.

Julie
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Ah, sorry. I just noticed you are using Project 2010. So -- the Window Split command is now View ribbon, Details check box. Also you'll need to display the Task usage view and add Peak Units to see the 13% assignment units.