I know from experience that it works just fine. I put in two outside gfcis this way at the house 2 doors down from me 3 years ago. But I also fed lights from the gfcis.
No problems yet.

How I see it is... if Gfcis compare the current at their hot terminal to the current at their neutral terminal, then what lies beyond isn't even readable. Proof of that is when you wire a plug off of from a Gfci's line. It doesn't protect that plug, cause it doesn't detect what's outside of it, unless you hook to loadside ofcourse.
Same principal as tapping a smaller wire to a BBH. The wire only sees what the heater puts out, not the entire circuit current

We have been told:
"The neutral carries the unbalanced current of a multiwire branch circuit" ...true
"Gfci's trip on a 4mA unbalanced current"...true

And that's where lots of people stop, I think... and a norm developed. Two different unbalanced currents thought of as the same?

Wouldn't this situation be the same as using a 2-pole Gfci breaker to feed two different branch circuits on a 3-wire?