We here found that while Meldrum's acid as the reactive warhead allows for the attachment of a single chemical modification on aldehyde-containing proteins, pyrazolone derivatives in combination with a phosphine nucleophile enable protein dual site-specific conjugation with the same or distinct moieties. These reactions are efficient and convergent under biocompatible conditions and allow access to protein bioconjugates with superior stability, homogeneity and flexibility. Our work expands the repertoire of bioconjugation chemistries and offers opportunities to construct bioconjugates with defined structure that have potential for medical and biomaterial applications.