We consider a network of pairs of nodes that perform simultaneous communications over frequency-selective channels. We assume that the whole frequency band is divided into a number of subbands, and each transmitter can only use one subband. Assuming that the network is geometrically infinite, we use the throughput as a measure of network performance. We consider the problem of allocating the nodes to the subbands so that the total throughput is maximized, under the constraint of fixed total spatial node density. The optimization problem turns out to be nonconvex. We investigate the detailed structure of the functions involved in the optimization and identify a set of properties of the optimal transmitters densities over the subbands. An iterative resource allocation algorithm with low complexity is derived. From the simulations, it is shown that the optimal solution obtained through the theoretical analysis is consistent with the one obtained through exhaustive search.