Unfortunately, such a scenario can't be implemented in the Customer Attributes M2 extension. To make an attribute relation work, both Customer Attributes should be displayed on the same page.
As an alternative option, you may configure two attributes to display either on the registration page or the shipping block so that both attributes can be seen in the same area. Below is an example of a Customer Attributes Relation.
1. Create a new attribute with the Single Radio Button type:
2. In the display options, set it to appear on the Shipping and Registration pages:
3. Create and/or configure another attribute that will be the child (dependent) field to the previously created one. Choose the same display options for this attribute.
4. In Customers → Customer Attributes → Manage Attribute Relations, create a relation where the first attribute is defined to be the parent one and the second attribute as the child one.
The relation works with the attributes on the same page: