Superimposed sine wave
The issue is you're not adding the two waves, you're taking the midpoint between the instantaneous value of each.

Basically, the resistor divider you have constructed has an output that is a function of the voltages at each end. In this case, since the two values are equal, the voltage divider basically splits the difference between the two values.

If you want to actually sum the inputs, rather then average them, you will need an active circuit (usually implemented using an op-amp).