Two is better than one: Subject DPs in Spanish third person plural arbitrary sentences

Spanish has an arbitrary construction with no overt subject and obligatory plural agreement on the verb. When a secondary depictive predicate adjective appears in this construction, the adjectival agreement is optionally singular for some speakers. This number agreement mismatch reveals that this construction involves two distinct subjects: an implicit argument agent as well as an expletive structural subject. I demonstrate that this agreement optionality on secondary predicates is a result of a process known as defective circumvention (Storment 2025b), by which a probe can conditionally Agree past a featurally deficient goal to obtain features from a non-deficient goal. In this construction, the deficient goal is the implicit argument (Collins 2024), and the non-deficient goal is a plural null expletive. I further show that the optionality in these sentences is a result of a configuration which fails to fully satisfy economy conditions in the syntax.