A new method for the data fusion of atmospheric vertical profiles, referred to as complete fusion, is presented. Using the measurements of the MIPAS instrument, the performance of the method is compared with those of weighted and arithmetic means. The complete fusion perfectly reproduces the results of the simultaneous retrieval with equal error estimates and number of degrees of freedom, while arithmetic and weighted means have relatively low vertical resolution and differ from the simultaneous retrieval by more than their errors. In addition the problem posed in this context by systematic errors is analyzed and alleviating procedures are considered.