Your latter point is approx correct. The shared throttle screw sets idle, with the AF screws set reasonably and the carbs synced.
The AF screws tweak the air/fuel mix mostly at idle but they do account for maybe 10% of the mix with the throttle open; so they have an effect over the entire rpm range. To start with they should be set so the plugs don't foul at idle, but also not be too lean. They should be set to the same # of turns, but I don't recall the recommendation off the top of my head.
I'd go for syncing the carbs first, once the engine is running reasonably with the default # of turns of AF screws. The sync and throttle screws interact strongly, so you'll probably be adjusting the idle screw while you adjust sync. Put another way, the sync adjust moves the butterflies relative to each other so you can get both cylinders pulling about the same vacuum (that being the balanced condition), but while you do that the idle screw is holding them open, so depending on how you adjust the sync, both butterflies may be held too far open by the screw so the idle is too fast, or one of the butterflies goes fully closed. I found the XS carbs balanced easily. It might be helpful to put a few extra turns on the idle adjust to get maybe 1500-2k rpm, which will make the butterflies open a bit. Once you have a balance then back the idle down to where you want it, and further adjustments will only be tweaks at that point.
My old Bandit needed some fine adjustment of the AF screws after rejetting so the plugs wouldn't foul at idle, but on the XS I left the AF screws set to the default recommended values- no sign of fouling or leanness. I suppose you could tune them if you have an O2 sensor in the pipes to find the sweet spot.
