I second Highliner's suggestion...do this when the fishing is slow, to cover more depths, and get more gear in the water, so you can experiment with different lures.
When the fishing is good, don't bother!
Problems I've found:
-- learning from your mistakes means tangled lines, fouled gear, and you spending a lot of time dealing with gear problems with no gear in the water...that is time wasted NOT fishing! [and in that case, better to just have the one line in the whole time]
-- it makes it just slightly more hassle to check the gear, so there's a tendency to leave it longer...not good if there's crap in the water to pick up on your gear or you're getting small bites that wreck the bait.
-- when you do get a hit, it's a bit more chaotic, which rod is getting bumped, getting the other one out of way, etc. So your chance of a hit are increased, but so are your chances of losing it.
I tend not to if there's strong current or wind, a lot of other boats around causing frequent turns, lots of kelp around. But if it's a calm, easy day and the fishing is slow...then what the heck, mix it up a bit. Dealing with the extra gear makes the time fly by and there's some added excitement when you do get a bite.
Oh yeah, and one last one: speaking for myself, stacking rods when by yourself is a bad idea! Chaos guaranteed.
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