In the context of most traffic codes, you are not impeding traffic by taking the lane when it is necessary to prevent unsafe passing (of a single file).

If the lane is too narrow for one bike and a car, it is too narrow for two so there is no harm although local jurisdiction and interpretation by law enforcement may vary with regards to riding double in the lane. If on the other hand there is safe room for one bike and a car, then I would agree that riding double is impeding traffic.