We report the synthesis of the first series of heterobivalent ligands targeting the putative heteromeric 5-HT2A/mGlu2 receptor complex, based on the 5-HT2A antagonist MDL-100,907 and the mGlu2 ago-PAM JNJ-42491293. The functional properties of monovalent and heterobivalent ligands were characterized in 5-HT2A-, mGlu2/Gqo5-, 5-HT2A/mGlu2-, and 5-HT2A/mGlu2/Gqo5-expressing HEK293 cells using a Ca2+ imaging assay and a [3H]ketanserin binding assay. Pronounced functional crosstalk was observed between the two receptors in 5-HT2A/mGlu2 and 5-HT2A/mGlu2/Gqo5 cells. While the synthesized monovalent ligands retained the 5-HT2A antagonist and mGlu2 ago-PAM functionalities, the seven bivalent ligands inhibited 5-HT-induced responses in 5-HT2A/mGlu2 cells and both 5-HT- and Glu-induced responses in 5-HT2A/mGlu2/Gqo5 cells. However, no definitive correlation between the functional potency and spacer length of the ligands was observed, an observation substantiated by the binding affinities exhibited by the compounds in 5-HT2A, 5-HT2A/mGlu2, and 5-HT2A/mGlu2/Gqo5 cells. In conclusion, while functional crosstalk between 5-HT2A and mGlu2 was demonstrated, it remains unclear how these heterobivalent ligands interact with the putative receptor complex.