Inspired by Karr's algorithm, we consider the summations involving a sequence satisfying a recurrence of order two. The structure of such summations provides an algebraic framework for solving the difference equations of form $aσ(g)+bg=f$ in the bivariate difference field $(\mathbb{F}(α, β), σ)$, where $a, b,f\in\mathbb{F}(α,β)\setminus\{0\}$ are known binary functions of $α$, $β$, and $α$, $β$ are two algebraically independent transcendental elements, $σ$ is a transformation that satisfies $σ(α)=β$, $σ(β)=uα+vβ$, where $u,v\neq 0\in\mathbb{F}$. Based on it, we then describe algorithms for finding the universal denominator for those equations in the bivariate difference field under certain assumptions. This reduces the general problem of finding the rational solutions of such equations to the problem of finding the polynomial solutions of such equations.