Similar to the conservation of energy in physics you can ask yourself "is there enough space for the information?". If somebody tries pulls two signals out of one signal, they better don't have too much entropy each. For a given bandwidth (number of independent samples), a given signal energy, and a given noise energy, there is a hard limit to the amount of information you can pull out.

If your signals are not "full entropy" or "white noise", you can do all sorts of funny tricks to increase the SNR to some extent. These tricks sure are nice but they veil the true issue at hand which is about information and capacity.