And the answer is "yes": Pteronarcys biloba and Pteronarcys dorsata in the very same stream

Checked with Steven Beaty on this one, and "yes," it is possible for two different species of Giant stoneflies to live in the same stream -- more, in the very same set of riffles in the same stream, the riffles pictured in the photo above.

Buck Mt. Creek: Pteronarcys dorsata, June 20, 2013

Buck Mt. Creek: Pteronarcys biloba, September 5, 2013

That's the most tolerant Giant, -- P. dorsata, TV of 2.4 -- alongside the most intolerant Giant -- P. biloba, TV of 0.0.

Beaty added that this is most common in "good" streams.  People should pay more attention to Buck Mt. Creek.  I've said it before: I've found a greater variety of aquatic insects in this particular stream -- orders, families, genera, species -- than in any other stream I explore, ranging from very intolerant to very tolerant.  (For the up-to-date taxa list for Buck Mt. Creek, see the entry posted on 6/20/13).