Filter Objects of coincident line

[sky]

I create two btConvexHullShape shapes,here are the two shapes by debug draw:

1111.png

they are separated as yellow color and blue color.
Bullet detects collision on the border, but this is not what I want,
here are the codes of collision world:
_collisionConfiguration = new btDefaultCollisionConfiguration();
_dispatcher = new btCollisionDispatcher(_collisionConfiguration);
_solver = new btSequentialImpulseConstraintSolver;
_broadphase = new btDbvtBroadphase();
_collisionWorld = new btDiscreteDynamicsWorld(_dispatcher, _broadphase, _solver , _collisionConfiguration);
_debugDrawer = new CollisionDebugDrawer();
_collisionWorld->setDebugDrawer(_debugDrawer);
if two shapes have a coincident border, I don't want they have a collision result.how should I do to avoid this?