The best way of attaching a halyard clip to a halyard is with an eye splice. We do a full eye splice with both core and casing spliced into the rope. It is the best way to put an eye in the rope without weakening the rope.
When cutting all rope we use a rope knife to seal the ends, however on all modern spectra and dyneems lines the core and casing will still seperate. An end whipping is the best way to stop the cassing and core from seperating.