Thanks for the suggestions. Sounds like some type of angled trim on the two lines and stitching OR the butt and paper-clip mode should work for my needs.
Regarding the Flemish Eye, in look online at how to create that it the site I found indicated this was not a strong eye and not intended to be strong. On the mainsheet there is a SS fitting that attaches to the sail. It's tied to the current line with a bowline, but if I wanted to splice a loop into this fitting when the new halyard is installed what type of eye should be done for that where there is a load.
Correct. A Flemish or Reaving Eye is used for 2 purposes: 1) finishing off the the end of the line so it doesn't fray no whipping or melting is necessary, and 2) making it easier to run lines through masts, booms and other places where access is restricted.
To attach a shackle an eye splice is used or some prefer using knots.
When both the old and new halyards have reaving eyes, halyard changes are easy and quick. On the old halyard attach a messenger line with a small knot and tape to secure it. Remove the old halyard by pulling the halyard up and through the masthead bringing the messenger line back down. Secure the tail of the messenger so it doesn't accidentally run up the mast and remove the old halyard. Attach the tail of the new halyard to the messenger line and pull back up the mast and through the masthead. I use ⅛" polyester braided line as a messenger line.
This method does not rely on tape or sewing for attaching the two lines, it is quick and easy. Any powerboats who are nearby will be amazed at the whole process.
If tape is used, I would not use duck tape. Use a good vinyl electrical tape or rigging tape. 3M makes a premium tape for about $3 at a big box HW store or $6 at marine store.