The concept of using multiple lines

[Jeebs]

I've read many posts about using more than one line to qualify a horse, which seems to be a fair application. One performance does not always do a horse justice. I've read Doc's guideline of when using multiple lines, to hide/eliminate the one scoring lowest on BL/BL. But what if you wish to keep both lines in play? There are old posts floating around (pre-2018) that either preach or give examples of when multiple lines were used, but offer little to no information on how to make a proper evaluation as a whole.