get position within sprite

Hello,

So I have a set of (possibly atlassed) sprites.
Now I want to draw a gradient so that it ranges from the bottom to the top of the individual sprites.
How is that possible? My shader works just fine if I have one sprite and I can just map vertex from
struct appdata
{
float4 vertex : POSITION;

to a color. If I have more than (strangely) 2 of the same sprite I seem to be getting a big world space mesh and now my gradient sits on the world axis...
Similarly, with an atlas I cannot use uvs.

So what can I do? Would I have to write my own sprite renderer that lets me feed in a second uv set? ugh.

Thanks for any hints!

seb