I tried something similar, and it does work as expected using betzaNew.js. But not with betza.js. Which does surprise me, as I had not expected it to be different in this respect, so I will look into that.
And make sure to set maxPromote=0 to disable the normal promotion by zone rather than by morph.
[Edit] It appears that multiple promotion sets were never implemented in betza.js.
I tried something similar, and it does work as expected using betzaNew.js. But not with betza.js. Which does surprise me, as I had not expected it to be different in this respect, so I will look into that.
And make sure to set maxPromote=0 to disable the normal promotion by zone rather than by morph.
[Edit] It appears that multiple promotion sets were never implemented in betza.js.