In the present article we study diffuse interface models for two-phase biomembranes. We will do so by starting off with a diffuse interface model on $\mathbb{R}^n$ defined by two coupled phase fields $u,v$. The first phase field $u$ is the diffuse approximation of the interior of the membrane; the second phase field $v$ is the diffuse approximation of the two phases of the membrane. We prove a compactness result and a lower bound in the sense of $Γ$-convergence for pairs of phase functions $(u_\varepsilon,v_\varepsilon)$. As an application of this first result, we consider a diffuse approximation of a two-phase Willmore functional plus line tension energy.

30 pages. v2: minor corrections