This invention relates to a double-plenum or double-chamber inlet manifold or splitter, made up of a structural unit comprising two distinct coupled plenums with a common wall, an inlet opening for each of the two plenums and several pipes through each of the said plenums, the said superimposed plenums having substantially flattened general structures and forming, with their respectively connected inlet openings and pipes, two independent circulation circuits running from the entrance at the inlet openings to the outlet at the external openings of the pipes, the said external or outlet openings of the pipes being grouped in pairs, with each pair having an opening of each of the two types of pipes; the manifold comprises two one-piece parts assembled together, in a gas-tight manner, at the continuous assembly areas, a first part incorporating at least the two inlet openings and first portions of the walls of the two plenums, contiguous to the said openings and the second part incorporating the pipes and second portions of the walls of the two plenums, contiguous to the said pipes and complementary to the above-mentioned first portions.