So the general consencus here is a big No. And if we're talking about a student who has just done some dual circuits with another instructor, and is now about to do some solo circuits immediately following that flight, then that makes sense.
But.....
Imagine a student about to go on a solo cross-country. The only other instructor (the student's regular instructor) is not available - maybe he's off on a dual cross-country himself with another student.
You review the student's training record. He has completed several hours of solo circuits. He has also done a couple of solo navigation trips prior to this one. His last trip was a dual navigation trip with his regular instructor, on the same route as he is flying today, and his instructor's comments were all positive.
You now talk to the student, go over his plog, question him about the weather, the Notams, the air traffic units he will contact, the altitudes he plans to fly at, etc. He has a good understanding of the route. His wind calculations are all correct. He maybe asks you a question about one particular aspect of the flight, and you answer the question. Then you send him off for his cross country.
Surely you are now supervising this solo flight, despite having never flown with the student?
FFF
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PS - Craggenmore, BEagle is correct that the QXC only counts as one flight, this is explicitly stated in LASORS