Multimode signal transmission concept is employed in this paper to increase the channel capacity of a substrate integrated waveguide (SIW) interconnect. In order to generate two uncoupled channels without adding an additional physical link, TE10 and TE20 modes of an SIW interconnect are harnessed as signal carriers. To excite these two orthogonal modes simultaneously, a multimode double launcher is proposed and characterized. It is shown that the performance of the launcher is limited by the coupling between the two mode launchers (transitions). Therefore, a method for improving port isolation is suggested, which results in increasing the channel bandwidths. A balun structure based on an SIW is presented and included in the multimode waveguide in order to efficiently excite the TE20 mode. Ultimately, the multimode waveguide with the optimized transitions is used in a parallel data transmission system demonstrating excellent transmission quality for the data rate of a 1-Gb/s/channel.