S21 parameters multiplication
It isn't that simple. There can also be an effect due to reflections back and forth between the input of L and the output of K.

If L has no input reflections (\$S_{11}=0\$) or K has no reverse reflections (\$S_{22}=0\$), then your formula should work. (Edit: As I think about it some more, you'd also need to have a perfectly matched load on the output of L)

But if that's not the case, you have to jump through more hoops. The usual approach is to transform the model to a different representation called "T parameters":

$$T_{11}=\frac{1}{S_{21}}$$
$$T_{12}=-\frac{S_{22}}{S_{21}}$$
$$T_{21}=\frac{S_{11}}{S_{21}}$$
$$T_{22}=\frac{-(S_{11}S_{22}-S_{12}S_{21})}{S_{21}}$$

The T parameters can then be cascaded:

$$T^{(KL)}=T^{(L)} T^{(K)}$$

And then the T parameters can be transformed back to S parameters:

$$S_{11}=\frac{T_{21}}{T_{11}}$$
$$S_{12}=\frac{(T_{11}T_{22}-T_{12}T_{21})}{T_{11}}$$
$$S_{21}=\frac{1}{T_{11}}$$
$$S_{22}=-\frac{T_{12}}{T_{11}}$$