Abstract The co-pyrolysis technology of the second-generation feedstocks has both engineering and environmental advantages towards resource recovery, waste stream reduction, and energy generation. However, there exists a large knowledge gap about the co-pyrolytic mechanisms, kinetics, emissions and products of biomass wastes. This study aimed to quantify the co-pyrolytic interactions between the five (N2, CO2, and three mixed) atmospheres and the two feedstocks of sewage sludge (SS) and coffee grounds (CG) as well as their emissions and products. Thermogravimetric-Fourier transform infrared spectrometry, two-dimensional correlation spectroscopy and pyrolysis-gas chromatography/mass spectrometry analyses were combined. An eight-parallel distributed activation energy model was adopted to elucidate the dynamic reaction mechanisms in the co-pyrolytic atmospheres. The co-pyrolytic interaction changed the maximum weight loss rate of the first peak by −2.5 to 38.6% and −1.9 to 36.9% in the N2 and CO2 atmospheres, respectively. The mass loss rate peak in the first stage was higher in the N2 than CO2 and mixed atmospheres, while the peak temperature of the maximum mass loss rate in the second stage declined with the elevated CO2 concentration. The replacement of N2 with the different CO2 concentrations significantly increased the activation energies of the 5th and 7th pseudo-components. The temperature dependency of evolved gases was of the following order: ethers/esters → acids/ketones/aldehydes/CO2 → hydrocarbons in the N2 atmosphere, and acids/ketones/aldehydes → esters/ethers → hydrocarbons in the CO2 atmosphere. The co-pyrolysis improved the yields of the hydrocarbon and phenol-type compounds and reduced the formations of the acid and nitrogenous compounds. Our results yielded valuable insights into a cleaner co-pyrolysis process.