Answer:
When two signals have frequencies that are spaced far apart, OptiSystem will automatically separate them into different channels, to save on computation time. When frequencies are very far apart the interactions between the signals are usually negligible, even in most non-linear cases.
However, you may force them to stay in the same channel with the Merge Optical Signal Band component. For example, see the figure attached. Before the merge, the two laser signals were considered in separate channels, visualizers interpret this as a discontinuity between signals. After the merge they are connected to the same channel and exist as a continuous signal.