how to mix two audio signals?

Simple mixing is just adding. 

y3 = y1 + y2;

If you want to control the level of each within the mix, then you need to adjust them as follows:

y3 = (y1*y1_level) + (y2*y2_level);

where yx_level is based on the decibel conversion:

yx_level = 10^(level/20);

So to make signal 2 half the amplitude of signal 1, you'd use -6 dB as follows:

y3 = (y1*(10^(0/20)) + (y2*(10^(-6/20));

All of this assumes that the two signals are the same dimensions and have the same starting level.