If you are doing this digitally you take the values of the two sinusoids at a paticular point in time, add the numbers together and send the result to your DAC.
If you have two seperate analog signals then it can be as simple as feeding both signals into one point through resistors.
Thanks,echo47.
But is seemed to be useless.
I want to know how to realize two sin signals add together.
To throwaway18
I confused with that the low frequency output signal will infect high frequency signal, when the two sin signals add together by resistor.
The point where the resistors are connected together is held at zero volts by the action of the opamp so there is very little leakage from one input to another.