I have the same question, I've built several custom components and they work in a isolated fashion in an Activiti Form (they can not see other form properties) neither you can get access to the field container, so you can perform validation between dependent fields, so i think this is not the answer.
In Vaadin is easy, because you use the "Form" class (container) that has methods to retrieve all the fields.
An easy way would be to have access to the container (the form) from the custom component instance. Then you are able to retrieve any other form property and if I select a value in a combobox, I can use this value to apply a filter for the values of the second combo box.
Please let me know if I'm wrong, but i think that currently there is no way for this sequential dependency betwen form properties.
Regards,
Eduardo Yánez.