Draw 3 lines.
Select 1, and use the Appearance panel to add a second stroke
Result: Works fine.
Select the other 2 lines, and attempt to add a second stroke
Result: Fails.
Original stroke is replaced by the new stroke.
Then original reappears, and second stroke disappears when selected.
UPDATE…
Now select either line on its own, and you will see that the second stroke was actually added, but not shown!