Separate

1. Create a polygon cylinder.
2. Change Subdivision Height to 3.
3. Look at the cylinder in the front view and change it to face mode.
4. Select the middle part and delete it. The separate two cylinders are still combined. You may change it to Object Mode and try to move them.
5. Select them and select Mesh > Separate. One cylinder is green and the other is pink. They are separated.