A route of synthesizing molecular bottlebrush polymers in a one-pot, one-step polymerization approach is presented. Through the combination of two orthogonal polymerization techniques, the backbone and side chains can be synthesized at the same time. Both polymerizations have to be compatible regarding mechanisms, chosen monomers, and solvents. Here, the mutual compatibility of the heat activated 2,4-dihydro-2,4,5-triphenyl-1,2,4-triazol-3-ylidene-catalyzed ring-opening polymerization with the ring-opening metathesis polymerization initiated by the third-generation Grubbs catalyst is demonstrated. Molecular bottlebrushes were synthesized with different lengths of poly(L-lactide) side chains and various poly(N-(hydroxylethyl)-cis-5-norbornene-exo-2,3-dicarboximide) backbone lengths. Their synthesis was achieved both in a one-pot grafting through approach as well as through the simultaneous ring-opening polymerization and ring-opening metathesis polymerization. The simultaneous bottlebrush polymerizations yielded short molecular bottlebrushes with excellent control and dispersities between 1.07 and 1.15. The reaction was followed by in situ H-1 NMR.