Bioconjugation chemistry is a subject of growing interest, especially for the preparation of antibody drug conjugates (ADCs). Selective introduction of bioorthogonal functionalities into the protein represents the main challenge in this research area. Enzymatic modification avoids complex manipulations of the translational machinery and only requires a short amino acid sequence in the protein of interest, which is recognized by the enzyme to incorporate a bioorthogonal functionality or to connect the payload to the protein. The formylglycine generating enzyme (FGE) is a valuable tool for bioconjugation. It catalyzes the oxygen-dependent oxidation of a cysteine residue within the recognition sequence CXPXR in type-I-sulfatases to Cα-formylglycine (FGly). This recognition sequence can be introduced into recombinant proteins and converted by FGE in vivo or in vitro. Eukaryotic and prokaryotic variants of FGE are known, and different catalytic mechanisms have been postulated. In addition, the ironsulfur protein AtsB of the radical-SAM (S-adenosyl methionine) protein superfamily is another FGly-generating system exclusively found in prokaryotes. Unlike FGE, AtsB oxygenindependently catalyzes FGly formation with a broader substrate scope also accepting proline-free motifs. Some AtsB variants even accept serine for FGly-generation. The aldehyde tag methodology has been proven highly efficient for the generation of site-specific ADCs with defined drug antibody ratios. However, the introduction of different payloads remains a challenge. In order to overcome this problem, we present a new aldehyde tag methodology making use of two different recognition sequences selectively addressed by different formylglycine generating enzymes.