Slit plant molder unit 12. Rest for controlling the inlet guide transverse position in the bar 14, the rolling unit 12, two cross-section to reduce the billet in the rolling section, has a two section two strands (1 and 2) cutting box (13 separating a), the two strands (1, 2) speed of the pair of downstream of one or more rolling units, cutting box (13) for rolling and / or cross-section measuring instrument (20 , 21), a shear (15). A method for controlling a plant that each cross-section of the two strands (1, 2) each of the rolling speed (V1 and V2) and / or the two strands (1, 2) downstream from a cutting box 13, the instrument of the pair It provides for measuring the (A1 and A2); In a continuous feedback stage, the two strands (1, 2) based on the plant to be operated to reduce the difference in length between, detects the lengths of the two strands (1, 2) and calculating the difference of its length do.