I'm interested in understanding how one may associate modular symbols to the L-functions and $p$-adic L-functions associated to the Rankin Selberg convolution of two modular forms/ elliptic curves and the symmetric square of a modular form/elliptic curves. Looking through the literature I see that the $\operatorname{GL}_{n-1}\times \operatorname{GL}_n$ case has been treated in general, for instance "Modular symbols for reductive groups and p-adic Rankin-Selberg convolutions over number fields"-Januszewski (arXiv/publisher link) and "Relative modular symbols and $p$-adic Rankin-Selberg convolutions"-Schmidt (behind paywall).

Is there a reference for the theory of modular symbols in the $\operatorname{GL}_{2}\times \operatorname{GL}_2$ and $\operatorname{Sym}^2$ cases?