We present a scanning tunneling microscopy (STM) study of the co-deposition of In and Sn in the submonolayer range (typical coverage: 0.1 ML) on the Si(100) 2 x 1 surface. The pure elements Sn and In are known to form one-dimensional (1D) lines of dimers on this surface. The aim of the present study was to examine the possibility to grow 1D metals by co-deposition of group III (In) and group IV(Sn) elements on the Si(100) 2 X 1 surface. We have analysed samples with different concentrations prepared at or slightly above room temperature. The position of the Sn and In atoms could be obtained from pairs of opposite bias images due to a strong contrast of electronic origin. The results show that metal atoms still form 1D structures, with In and Sn atoms co-existing in the lines. Measurements of the apparent height of the various structures indicate, however, that "mixed" In-Sn dimers are scarce in our growth conditions. These results will be discussed in connection with ab-initio total energy calculations of the possible structures of the dimers.