That would not be true the given that we see twin borns in the 2nd era being you know born naturally.
It is quite possible for there to be a twin born be a combination as follows beyond the single twin born powers
Full mistborn full ferochemist
Full mist born single power ferochemist
Single allomantic power full ferochemist
The unsettling issue in the 2nd era is did harmony stop full ferochemist and mistborn from being born. As it does seem unnatural that neither have been born since sazed saved the world.
