You can't. Invariants have access to values for other fields in the same interface, but not fields from other interfaces.
You can use a widget manager validator instead: http://developer.plone.org/reference_manuals/active/schema-driven-forms/customising-form-behaviour/validation.html#widget-manager-validators
Or do the validation in your form's action handler: http://developer.plone.org/reference_manuals/active/schema-driven-forms/customising-form-behaviour/validation.html#validating-in-action-handlers