How does the acquisition of semantically complex expressions track the acquisition of their constituent meanings? We investigate this question using the English quantifiers both and either. These quantifiers, while morphologically simplex, are semantically complex, comprising of two pieces: (i) universal/existential quantification and (ii) a size restriction on the quantificational domain to 2. Across two experiments, we compared the acquisition of these quantifiers with expressions mapping conceptual pieces that contribute to their make-up (two, all, any). Our results suggest that having all of the parts is not enough to put together the whole, a finding that could have implications for quantifier learning more broadly.