  We explore the interaction between two genetic incompatibilities
(underdominant loci in diploid organisms) in a population occupying a
one-dimensional space. We derive a system of partial differential equations
describing the dynamics of allele frequencies and linkage disequilibrium
between the two loci, and use a quasi-linkage equilibrium approximation in
order to reduce the number of variables. We investigate the solutions of this
system and demonstrate the existence of a solution in which the two clines in
allele frequency remain stacked together. In the case of asymmetric
incompatibilities (i.e. when one homozygote is favored over the other at each
locus), these stacked clines propagate in the form of a traveling wave. We
obtain an approximation for the speed of this wave which, in particular, is
decreased by recombination between the two loci but is always larger than the
speed of "one cline alone".
