I suspect they did that in part, but it's also critical to keep the signals distinct precisely because you expect the noise to be uncorrelated between the two s

I suspect they did that in part, but it's also critical to keep the signals distinct precisely because you expect the noise to be uncorrelated between the two signals so you're introducing an additional square root of two gaussian noise factor in the combination. This sort of thing is much more about statistics and error propagation than one encounters in more traditional signal processing.