As a brass player myself, I'd like to extend some tips for learning how to perform multiphonics on brass instruments. These tips are presented in a specific sequential order.
1.) Practice buzzing your lips while humming (without the instrument)
The first thing you need to do is begin getting use to the sensation of doing those two things at the same time (it's more difficult on the instrument). Practice humming while buzzing until you can do it without much mental effort.
1a.) Add your mouthpiece
Optionally, once the buzz/hum concept is understood, you can add the mouthpiece alone to begin simulating the effect for how your lips feel when buzzing while humming into a mouthpiece.
2.) Sing through your instrument with mouthpiece, without buzzing.
Sing throughout your entire range. Where possible, try and sing the overtone series for the pitch of your instrument. Feel / hear how your vocal frequencies "slot" into the appropriate tones corresponding to the overtone series. This step is important because you feel a similar "slotting" feel when performing multiphonics.
2a.) Optional: Just for fun, change valve combinations and sing overtone series for each different note.
3.) First note: I highly recommend your first multiphonic to be either a unison or an octave, depending on your instrument. A tubist would likely prefer and octave, a trumpet player likely a unison. The first multiphonic should be an "open" one, that is, without valve / slide combinations. Continue practicing this interval until you can hear it and reproduce it.
3a.) Repeat step 3, but now seek to sing/play unisons/octaves through all valve/slide positions.
4.) Repeat step 3, but this time attempt to sing a perfect-fifth higher than the note you are playing. ^^NOTE^^ It is typically easier to begin playing first, and then add the singing after.
4a.) Sing a perfect-fourth below the played note. Once interval is secure, slur down to a perfect-fifth below played note.
5.) Sing major third / tenth above played note. Again, repeat all of these until they become easy.
6.) Sing major sixth above played note.
7.) Sing major seconds and sevenths above played note.
^^NOTE^^ It is easiest to play multiphonics either monophonically or homophonically with parallel motion. The other types of motion: similar, contrary, and oblique, are more difficult and required additional practice because they require a modicum of independence between your vocal chords and your lips. Moving intervals in parallel motion requires less independence and is therefore more accessible.
^^NOTE^^ Practicing multiphonics is like anything else: riding a bike, patting head / rubbing tummy, 3:2 or 4:5 polyrhythms, or whistling - you really just need to mess around with it until you get it. Be patient, give it time (especially when you sleep between practice sessions - it's the best way to learn new things.)
Good Luck.
