The lines with dual fibers are interwoven, you can't separate them, but you get the idea.

Core removal is the easiest.

Assume you have a 15 foot section of line which is never heavily loaded. Measure from the traveler end and mark it. Milk the cover splice open to get to the core. Cut the core in one spot.

Milk the core out from the blocks end 15 feet.

You now have a 15 foot stripped tail also.

The ends should be sewn tight.