I figured that using a funny embouchure and fine control, it is possible to play multiphonics on the trumpet, especially in the high register. The sound is unstable and not very musical, just like any other wind instrument multiphonics. But I guess a good composer out there might find a good use for this unique voice, and a good player might figure out how to do it reliably.
Listen to it here:
If you don't get it, it is the vibrating sound between the two notes. It is unstable as it easily drops into either one of the more stable tones. But when everything is correct, the funny vibration comes. I know it sounds weird because of the funny embouchure involved, but it is an ordinary Bb trumpet, a Constellation, not a clarinet or some weird custom-made instrument. And, No! It doesn't involve playing while singing the 2nd note.