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).
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