Process for the combined preparation of butene and octene from ethene, which comprises the following steps: a) provision of a solvent whose boiling point or boiling range is above the boiling points of the butenes and below the boiling points of the octenes and which solvent is an inert solvent or is hexene alone or is hexene admixed with pentane or hexane or heptane or is a mixture of pentane, hexane, heptane; b) provision of a first feed mixture containing at least the solvent and ethene dissolved therein; c) provision of a second feed mixture containing at least hexene, the solvent and also ethene dissolved in the solvent and/or in the hexene; d) transfer of the first feed mixture into a first synthesis and of the second feed mixture into a second synthesis, where the first and second syntheses are physically separate from one another; e) oligomerisation of at least part of the ethene present in the first feed mixture in the presence of a first heterogeneous catalyst and in the presence of the solvent in the first synthesis to give a first reaction mixture comprising at least the solvent, butene and hexene; f) separation of a butene-containing low boiler fraction from the first reaction mixture or from a stream based on the first reaction mixture; g) separation of an intermediate boiler fraction containing hexene and the solvent from the first reaction mixture or from a stream based on the first reaction mixture; h) use of at least part of the intermediate boiler fraction in the course of the provision of the second feed mixture; i) reaction of at least part of the ethene present in the second feed mixture with at least part of the hexene present in the second feed mixture in the presence of a second heterogeneous catalyst and in the presence of the solvent in the second synthesis to give a second reaction mixture comprising at least octene and the solvent. a) provision of a solvent whose boiling point or boiling range is above the boiling points of the butenes and below the boiling points of the octenes and which solvent is an inert solvent or is hexene alone or is hexene admixed with pentane or hexane or heptane or is a mixture of pentane, hexane, heptane; b) provision of a first feed mixture containing at least the solvent and ethene dissolved therein; c) provision of a second feed mixture containing at least hexene, the solvent and also ethene dissolved in the solvent and/or in the hexene; d) transfer of the first feed mixture into a first synthesis and of the second feed mixture into a second synthesis, where the first and second syntheses are physically separate from one another; e) oligomerisation of at least part of the ethene present in the first feed mixture in the presence of a first heterogeneous catalyst and in the presence of the solvent in the first synthesis to give a first reaction mixture comprising at least the solvent, butene and hexene; f) separation of a butene-containing low boiler fraction from the first reaction mixture or from a stream based on the first reaction mixture; g) separation of an intermediate boiler fraction containing hexene and the solvent from the first reaction mixture or from a stream based on the first reaction mixture; h) use of at least part of the intermediate boiler fraction in the course of the provision of the second feed mixture; i) reaction of at least part of the ethene present in the second feed mixture with at least part of the hexene present in the second feed mixture in the presence of a second heterogeneous catalyst and in the presence of the solvent in the second synthesis to give a second reaction mixture comprising at least octene and the solvent.