The invention relates to a two-shaft compulsory mixer (1) whose mixing tools (4) are situated on two parallel mixer shafts (3a, 3b) in such a way and whose mixer shafts (3a, 3b) are also synchronised in such a way that with each rotation of the shafts (3a, 3b), the mixing tools (4) of both mixer shafts (3a, 3b) run from bottom to top through the horizontal line between the shafts (3a, b), axially adjacent to each other, in pairs, successively. In this way, the mixing tool pairs produce essentially continuous formations (9) as they pass through the area between the mixer shafts (3a, 3b), said formations emerging in a vertical direction between the two shafts (3a, 3b) from bottom to top. This significantly reduces the mixing times for a particular mixing quality compared to known two-shaft compulsory mixers with an identical number of middle mixing planes (5a-5h).