Zeal Items should be like Hextech
They should only be able to build one. Their unique passive does not work and only one will apply at a time. However phantom dancer will be annoying to balance, so making it so that they could only build one is only fair.
Hextech items have active and cds are shared between all 3, so it is only natural for ADC having restrictions from buying more than 1 zeal item as well.