We evaluate the electron scattering properties in CoSi thin films with having either twin (heterochiral) or non-twin (homochiral) grain boundaries using first-principles quantum transport calculations. We find that scattering across a twin grain boundary is more resistive than scattering across a non-twin grain boundary and that non-twin grain boundaries have a greater percentage of surface conduction than twin grain boundaries. This is corroborated by the transmission eigenstate profiles, which show that the surface Fermi-arc electrons can transmit past the non-twin grain boundaries between homochiral domains on the same surface without significant backscattering. In contrast, these surface electrons get scattered along the twin grain boundaries between heterochiral domains to the opposite surface, resulting in partial forward transmission and partial backscattering that yields higher grain-boundary resistivity. This work helps elucidate the complexities of electron transport in chiral topological semimetals for potential application in next-generation nano-interconnects.