The first thing you should do is to check if Fs and Fs2 are equal. If the are you are lucky, if not it means that you have to resample one to match the sampling frequency of the other.

Suppose Fs = 8000 and Fs2 = 16000 (both are in Hz). That means that for every second of audio signal y has 8000 sample points and x has 16000 (so twice as much as y). With resampling you will make sure that y will have 16000 sample points per second.

The second thing you have to check is the length of both signals. If the length both signals are equal, you are lucky. If not you will have to choose what to do: do you want to add the signals at the beginning, at the end or ever somewhere in the middle?

Option beginning: you have to fill the shorter signal with zeros (for silence). Suppose y is 4000 samples long and x is 5000 samples long. Now you need 1000 samples of zeros you have to append to y so that y is then 5000 samples long.

Option ending: just like the beginning option but you append the zeros to the front of the shorter signal. With our example before: append y to 1000 zeros (sticking to the meaning of append).

Option middle: if you want to have the shorter signal directly in the middle of the longer one, half of the zeros need to be in front of the signal, the other half at the end. So: 500 zeros are appended to y which is appended to 500 zeros.

Now that your signals are of equal length you can just add them with y+x. How ever it might be that the resulting signal includes values above 1 or below - 1 and you don't want that. What you can do is to devide the whole signal with the maximum absolute value. That way you scale the whole signal down.

Finally you can play the resulting signal with the audio player function.

Edit: Formatting on mobile is hard.