Offset path within appearance palette
It would be good to have 'offset path' built into the appearance palette, so you could automatically draw a stroke which was a double line, maybe of two different colours or widths, and it would perfectly shape itself around corners and bends.
Debbie, the purpose of the Appearance panel is to control the appearance, and not to store the complete graphics styles — that’s what the Graphics Style panel is for. You can build whatever appearance you want and save as a graphic style it for future uses.
And if you want the certain style to be automatically applied for the next shape you create — make sure you uncheck the 'New art has basic appearance' option in the Appearance panel flyout menu. This will ensure the next path gets the appearance form the object you had selected last.
You can also redefine the basic graphic style, by dragging a path with the appearance you want to make the new default with Alt/Opt held over the [Default] item in Graphic Styles — then, when you hit D, the selected art will get this new style instead of ordinary 'black and white' stroke and fill.
Finally, there are many way to create the double line you speak of. Here is the file with just two examples: one uses the transparency and knockout trick, while the other one (with colors) just has a brush.
Both are saved as styles already, and can be modified the way you like them.
Does this suffice? If not — please explain a bit more about the case you have: what do you try to get and why and how does you idea may help you with it?