The adjustment screws aren't so much an idle adjustment screw as they are a fine tuning mixture screw for the lower half of the rpm range. As an example, if they are set at 1 1/2 turns, idling fine, and you turn them to 1 turn, there may be no idle difference. But the difference it makes will be felt in the acceleration up the the 5K range where the main circuit takes over. Loss of power or hesitations will be felt. 2 turns or 2 1/2 turns out seems to be the optimal setting for most carb sets. But I have encountered several that needed to go 3 turns to smooth out any issues.
The way I test the mixture screws and the low circuit is to turn them in until I hear the idle changing and note where that is before I turn it back out to 2 or 2 1/2 turns. Then I test the other side to see that it responds in the same amount of turns in. Sometimes I don't get an idle drop until it's around 1/2 turn out. But as long as they are near the same I know the circuits are clean and matched on both carbs.
I start road testing at 2 1/2. If everything responds well, I'll turn it in 1/4 of a turn and ride it again. If that runs fine I'll turn another 1/4 in. I do this until I feel a difference in power then I'll back it out 1/4 to 1/3 and leave it there. There's no need to run them out any more than they need to be. A little rich is ok, but a little lean will be felt in power. This is why a plug will look black after sitting in the garage idling for a while while we work on the bike. They won't show the true color of the mixture until you take it out and ride it under a load. The plugs will reach full heat cycles that way and burn off any black soot that was built up while the bike sat running under no load. It may take 5 miles or more of riding to reach the heat range of the plug before you'll see the actual end results.
It sounds like your set is clean and at it's best since they started to falter at 1/2 turns. Road testing around the neighborhood under 5K will tell you where the mixture screws need to be at for the bike to run it's happiest.
Hope this helps explain why the screws didn't act like the single cylinder bike.