service road or track?

What is the proper tag to apply to a dirt double-track farm road? Is there a way to create paralell tracks to designate a double track road?

I would go with the Track and then set tracktype.

You can add the lanes tag to specify if it is double track. ie lanes=2 if it is wide enough for 2 cars to pass.
Or you can add the width tag for the width in metres.