Calculating relative speed with TLE

Orekit usage
1

Hello everyone.
I want to calculate relative speed of two satellite with TLE. I propagated the satellites and extract velocity as below but the result is not correct

  // Extract the velocity vectors
            Vector3D velocitySat1 = stateSat1.getPVCoordinates(inertialFrame).getVelocity();
            Vector3D velocitySat2 = stateSat2.getPVCoordinates(inertialFrame).getVelocity();

            // Calculate delta-v
            Vector3D deltaV = velocitySat2.subtract(velocitySat1);