Re: double hauling on a stream/river
I am confused by your posting. Double hauling is a technique to gain line speed during the aerial casting stroke (back cast & forward cast). It has nothing to do with affecting the drag on the line due to conflicting currents in the water.
I can see that an effective double haul could allow an angler to reach farther spots on the water which could lead to the fly and the remainder of the line being in conflicting currents.
I tend to avoid fishing long distances due to the above issue. However, I have used a double haul to gain accuracy (higher line speed = less effect from any wind gusts) while driving fly under a low tree branch across the stream. But such cases tend to be limited to streamers where drag is not a big issue (unless one is dead drifting them).