If it is safe for a car to pass you within the lane if you're going single file, then IMO (if it's safe) you should probably go single file.
If they can't pass a single file line within the lane anyway, then they have to change lanes to pass and you're being LESS of an inconvenience by riding two abreast (you're giving them a line half as long to pass).
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Work: the 8 hours that separates bike rides.