> If you do separate, parallel yes-or-no referendums on both A and B (as frequently happens in California), it is likely that both would pass, resulting in a sub-optimal outcome as practically no one really wanted both.

Somewhat redundant measures in California that are proposed at the same time generally are legally conflicting, which means that if both are passed, only the measure with the greatest margin actually goes into effect. I can't think of any real examples of simultaneous, partially-redundant measures that both had majority support and had no legal conflict, so that both went into effect, where you could make a plausible case that the kind of problem you describe was in play. Can you present any?