By tile im assuming you mean 1 quad (2 triangles) right?
If so, you could;
1) track down those 4 vertices coords and add new surface geometry to texture with the other texture,
or:
2)use 2 texture indexes on the same brush with separate UV sets (however there are only 2 sets of UVs per brush and 1 brush per surface AFAIK so doing this would make lightmapping difficult) This method is not geometry bound like the other so you could use the UVs to position it anywhere you like.
However, i notice you are talking about rippling grass. Why not just use crossplanes with a png alpha texture on certain vertex positions? You could make the system dynamic by using copyentity and freeentity when in and out of camera range/view.
For the copyentity you would use a sourcemesh which never gets freed and is invisible (use hideentity) but copies itself where and when needed.
If so, you could;
1) track down those 4 vertices coords and add new surface geometry to texture with the other texture,
or:
2)use 2 texture indexes on the same brush with separate UV sets (however there are only 2 sets of UVs per brush and 1 brush per surface AFAIK so doing this would make lightmapping difficult) This method is not geometry bound like the other so you could use the UVs to position it anywhere you like.
However, i notice you are talking about rippling grass. Why not just use crossplanes with a png alpha texture on certain vertex positions? You could make the system dynamic by using copyentity and freeentity when in and out of camera range/view.
For the copyentity you would use a sourcemesh which never gets freed and is invisible (use hideentity) but copies itself where and when needed.