According to the National Conference of State Legislatures, “Forty state constitutions contain a provision that requires a bill to address or contain a single subject.” The principle is called germaneness. While this seems like a sensible rule, there are cases in which independent evaluation of decisions can produce worse outcomes.
For example, suppose our voting blocs have the following preferences regarding independent decisions X and Y. (Capitalization indicates the presence or absence of a provision, so “XY” means the bill contains both provisions X and Y, whereas “Xy” means X is present but Y is absent.)
Bloc size : Preference
35% : Xy > XY > xy > xY
33% : xY > XY > xy > Xy
32% : xy > others
Note that a large 68% majority would prefer the bill XY to pass vs. the status quo of neither X nor Y. However, if these provisions are voted on independently, X fails by a 65% to 35% majority vote (2nd and 3rd rows). And Y fails by a 67% to 33% majority vote (1st and 3rd rows).