If we want to show stability for a scheme like this: \begin{align*} p_{n+1}&=y_n+\frac{1}{2}h[3f(t_n,y_n)-f(t_{n},y_{n})]\\ y_{n+1}&=y_{n-1}+\frac{h}{3}[f(t_{n+1},p_{n+1})+4f(t_{n},y_{n})+f(t_{n-1},y_{n-1})]\\ \end{align*} Do we then have two errors $\varepsilon_n$ in the perturbed form
\begin{align*} p_{n+1}&=y_n+\frac{1}{2}h[3f(t_n,y_n)-f(t_{n},y_{n})]+ \varepsilon_{n,1}\\ y_{n+1}&=y_{n-1}+\frac{h}{3}[f(t_{n+1},p_{n+1})+4f(t_{n},y_{n})+f(t_{n-1},y_{n-1})]+ \varepsilon_{n,2}\\ \end{align*}
Or only one in the second one:
\begin{align*} p_{n+1}&=y_n+\frac{1}{2}h[3f(t_n,y_n)-f(t_{n},y_{n})]\\ y_{n+1}&=y_{n-1}+\frac{h}{3}[f(t_{n+1},p_{n+1})+4f(t_{n},y_{n})+f(t_{n-1},y_{n-1})]+ \varepsilon_{n}\\ \end{align*}
Thank you for the answer.