Calculating RMS/Crest Factor for a Stereo Signal
Conceptually you can think of it as creating one mono signal by concatenating the left and the right channel signals, and then computing the RMS and the Crest factor of that mono signal. With $N$ the number of samples in the current frame, and $x_l[n]$ and $x_r[n]$ the left and right channel signals, respectively, you get the following formulas:

$$\begin{align}x_{\text{rms}}&amp;=\sqrt{\frac{1}{2N}\left[\sum_{n=0}^{N-1}x^2_l[n]+\sum_{n=0}^{N-1}x^2_r[n]\right]}=\sqrt{\frac12\left(x^2_{l,\text{rms}}+x^2_{r,\text{rms}}\right)}\\
C&amp;=\frac{\max\{|x_l[n]|,|x_r[n]|\}}{x_{\text{rms}}}\end{align}$$

where $x_{l,\text{rms}}$ and $x_{r,\text{rms}}$ are the RMS values of the left and right channel, respectively.