Dear Susana,
There are differences between the single drive and dual drive MZ Absorption-Phase modulators. Please refer to the datasheet of each component for more information on the theory used in each component. Note that for the single drive you connect either voltage V1 or V2 to ground, which will affect the overall modulation response shown in the equation in the datasheet.
I don’t mind checking your project layout while using both components to address the difference. Please send me your project as a zip file to my email ahmad.atieh@optiwave.com
Regards,
Ahmad