Guys, I think I've found the fix if you're *just* trying to get multi-output working on a single set of stereo speakers (e.g. the built-in ones) without crackle/distortion. It's just the "Drift Correction" checkbox -- UNCHECK that sumbitch! It may as well be labeled "Add Crackle".
Also, actually don't use "Aggregated Device" -- use "Multi-output Device" instead: then [with Multi-out] you get both L & R channels and no "Speaker Configuration" is necessary.
I think the reason the "Aggregated Device" route doesn't work is because it seems it doesn't combine two streams into one. So it's like "hey I've got 4 streams to output (L&R x 2 sources), but only 2 slots" -- that would explain why most of us had the "only getting output to 1 device" problem. It's [presumably] because the L&R output for device 2 is going to channels 3 and 4 which we don't have. This would *also* explain why the "left front to 3, right front to 2" seems to work: then Device 2 receives the Left channel of our output while Device 1 receives the Right.
When I tried this "aggregate - stereo" method myself, I noticed that while my recorded audio was no longer distorted (because I had unchecked that vile, godforsaken box), I was only hearing it on the Left channel. After dumping the Aggregate Device and setting up a new Multi-Output Device again, I then did finally at long last have non-crackling audio going to *both* devices and *both* L&R channels. Whew.
Also, googling "Drift Correction crackle" seems to show this is a very common problem, and that unchecking it was all that was needed.
