Vertex collision of sub-meshes safe?

Hello, When I learned to use Maya (that 3D modelling software), I was told that vertex, edge and face collisions were bad. For vertices, if there were two vertices at the exact same position or very close, it would be better to merge the vertices.

Does the same apply to Unity's mesh filter? I ask because I have this situation: I have a mesh renderer with two materials. I want to create a two sub-meshes to render the materials separately. However, the faces of the two sub-meshes have to touch.

So, the vertices and edges would look something like this:

 A---B---C
 | / | / |
 D---E---F

Now, B and E are the shared vertices. The problem is that I want the first face to have different UV co-ordinates from the second face. That means B and E would have to have two different UV co-ordinates each.. And that's not possible.

So, this is what I'm doing now:

 A---B G---C
 | / | | / |
 D---E H---F

B and G have the same position co-ordinates. E and H have the same position co-ordinates, so, in reality, they have collided.

This renders fine and does exactly what I need but I'm wondering if this is safe because, when I learned 3D modelling, I was told that this was very bad and could crash certain rendering engines (namely, the old ones).

And if this is wrong, how would I achieve what I want the correct way?