I have two raw sound streams that I need to add together. For the purposes of this question, we can assume they are the same bitrate and bit depth (say 16 bit sample, 44.1k
I'd say just add them together. If you're overflowing your 16 bit PCM space, then the sounds you're using are already incredibly loud to begin with and you should attenuate them. If that would cause them to be too soft by themselves, look for another way of increasing the overall volume output, such as an OS setting or turning the knob on your speakers.