I've seen a GTM with a dual intake, with a MAF in one of the tubes. How did it work? One of the tubes was closed off, so you had the appearance of two tubes, even as only one was actually open. IOW, only the tube with the MAF actually flowed any air to the engine.
Perhaps that is the one you saw?
I don't think that one met the standard in the GMPP document, though, as there wasn't a straight of the correct length before or after the sensor.
It would be interesting to know whether or not the owner experienced driveability issues, as that is the likely consequence of turbulent air past the MAF sensor (which the straight lengths minimize).