The present invention relates to a method for the synthesis of compounds useful in the preparation of antibody drug conjugates (ADC), namely, monoprotected dimeric bifunctional prodrugs based on duocarmycin analogs. In a further aspect, compounds obtained by the method according to the present invention are provided. The monoprotected bifunctional prodrug is used for preparing antibody drug conjugates composed of an antibody moiety and the monoprotected bifunctional prodrug. The antibody compound conjugates thus obtained are provided. Further, a method of preparing an antibody drug conjugate composed of two identical or two different antibody moieties is provided as well as the antibody compound conjugate containing two different antibody moieties accordingly. These conjugates can be used in pharmaceutical compositions, in particular, for use in treatment of tumors, e.g. for use in ADC therapy.