The invention relates to a process and to an installation for preparing a final glass, comprising a main furnace with electrodes and/or overhead burners, which is fed with main batch materials generating a main molten glass, and a submerged-combustion auxiliary furnace, said auxiliary furnace being fed with auxiliary batch materials, the auxiliary molten glass feeding the main furnace toward its upstream end in the first third of its length, the auxiliary glass being substantially of the same composition as the main glass. The downstream zone of the main furnace is thus used to remove both gases coming from the main glass and gases coming from the auxiliary glass, in order to finish melting the batch stones and impurities contained in the auxiliary glass and to homogenize the two glass streams from their redox standpoint, when this is necessary.