In this case it could be possible to run two separate simulations with one of the sources in each simulation. Then only 1 k vector is needed in each simulation so there won’t be any conflict in the boundary conditions settings. You could then sum the fields from the two simulations to get the result as though both sources were injected at once.
This is demonstrated in the following example which shows how to simulate a circularly polarized plane wave source:
In the example, two methods are used to obtain the same final result. In one method two plane wave sources with orthogonal polarization and a 90 degree phase shift between them are included in the same simulation file. In the second method each source is simulated separately and the sum of the results is the same as the first method.