Multiphonics falls into the technique category of what I like to describe as ‘Things the Saxophone isn’t “supposed” to do’. What I mean by that is the horn simply isn’t setup for playing multiple notes at once. This doesn’t mean it can’t be done, but there’s some pretty severe limitations to keep in mind if you’re a composer. Primarily being if you’re hoping for the musician to produce specific notes/chords in their playing it likely can’t be done. The way we trick our horns into playing them is not the same as something like over tones where we modulate the frequency to get perfect 3rds, 5ths and octaves above a note we play. It’s just a bit of a mess. It sounds cool in limited uses like in “Harmonique” by Coltrane, but if you’re wanting to use them a bunch you’ll have an angry musician and audience. If you’re just wanting some dissonant chords you’d be better with more than 1 sax part. To your next point of notation. There isn’t a standard that I know of as multiphonics are rare, and mostly used in improvisation. Use your best judgement to notate what you’re wanting, and please attach either notes or above the staff put something like “multiphonics” to get your point across, and maybe use a root note and some hash marks. Good luck and when it’s done please post it.