A covalent adduct of DFOB and DOTA separated by a l-lysine residue (DFOB-l-Lys- -DOTA) exhibited remarkable regioselective metal binding, with {H}-C NMR spectral shifts supporting Zr(iv) coordinating to the DFOB unit, and Lu(iii) coordinating to the DOTA unit. This first-in-class, dual-chelator theranostic design could enable the use of imaging-therapy radiometal pairs of different elements, such as Zr for positron emission tomography (PET) imaging and Lu for low-energy β-particle radiation therapy. DFOB-l-Lys- -DOTA was elaborated with an amine-terminated polyethylene glycol extender unit (PEG4) to give DFOB- -(PEG4)-l-Lys- -DOTA (compound D2) to enable installation of a phenyl-isothiocyanate group (Ph-NCS) for subsequent monoclonal antibody (mAb) conjugation (mAb = HuJ591). D2-mAb was radiolabeled with Zr or Lu to produce [Zr]Zr-D2-mAb or [Lu]Lu-D2-mAb, respectively, and PET/CT imaging and / biodistribution properties measured with the matched controls [Zr]Zr-DFOB-mAb or [Lu]Lu-DOTA-mAb in a murine LNCaP prostate tumour xenograft model. The Zr-immuno-PET imaging function of [Zr]Zr-D2-mAb and [Zr]Zr-DFOB-mAb showed no significant difference in tumour accumulation at 48 or 120 h post injection. [Zr]Zr-D2-mAb and [Lu]Lu-D2-mAb showed similar biodistribution properties at 120 h post-injection. Tumour uptake of [Lu]Lu-D2-mAb shown by SPECT/CT imaging at 48 h and 120 h post-injection supported the therapeutic function of D2, which was corroborated by similar therapeutic efficacy between [Lu]Lu-D2-mAb and [Lu]Lu-DOTA-mAb, both showing a sustained reduction in tumour volume (>80% over 65 d) compared to vehicle. The work identifies D2 as a trifunctional chelator that could expand capabilities in mixed-element radiometal theranostics to improve dosimetry and the clinical outcomes of molecularly targeted radiation.