You know, I've never done much vertical fishing with jigs, except under
a float - only used one then, but don't really know why. But I thought
it was interesting how, when I used to side-pull, I ended up with a real
similar set up to what you're using. 18" apart was my ideal spacing too.
And I always used either a 1/8 above a 1/4 - or 1/16 over 1/8.
I can't really explain it, that's just what seemed to work best for me.
I didn't really know about dropper loops then, so I just used a tiny split
ring as a "junction" - main line, about a 3" dropper, and another dropper,
15-18" all tied to it. I was using Fireline main, and mono droppers, so
that worked good to join them together. Sometimes I have had trouble
with splice knots failing, going from Fireline to small diameter mono directly...
Shoals Area Crappie Association