Completed tasks
 

I prefer to put my "next action" items in two places: on a "next action" list under a context such as "@ home" and also in the folder that lists the steps that need to be taken to complete each project.

Is there any way to set my preferences so that when I check the action as 'done' in one place, I don't have to go to the other place and do the same--in other words, Omnifocus will somehow know that it's been done?

Kind of like with a Mac, you can make a change to a file on your hard drive, and naturally, the same change will be reflected when you open the file's alias on your desktop.

Thanks!

You don't create a next action list in OmniFocus. In planning mode you create the project list. You assign each action under the project a context. To view the next action list, switch to Context mode.

Have you watched the videos on the OmniFocus website?