Sprites consisting of two packed textures

Hi,

I'm creating a simple shader for an in progress painting. The base texture is treated as an alpha mask, and the secondary texture is the full painting.

I am referencing some pretty big png files, and using Unity to pack the opaque areas into one atlas. Using unpacked files I can simply share the UVs in the shader, but once the sprites have been packed, the UVs are no longer shared.

The question: How do I access the UV coordinates for the second (also packed) texture from a custom sprite shader. I can only seem to access the first set of UVs (pointing correctly to the packed base texture)

Below is my current input to the vertex shader.

             struct vertin {
                 float4 vertex : POSITION;
                 float4 uv_MainTex : TEXCOORD0;
                 float4 uv2_Unmasked : TEXCOORD1;
                 fixed4 color : COLOR;
             };

The rest is just regular vertex and fragment shaders, using the alpha from base and color from second texture.

Edit: Realized that I'm not targeting the secondary sprite, but rather the texture containing it. Is there a way to target two sprites? Or would I need to create my own sprite renderer?