A frequently asked question regarding the ASF (Ambidextrous Selector, FCD) levers configuration is, which lever for which side.
In short, while there are commonly recommended levers configuration for most users, the answer depends on the user's preference.
If you are a right hand shooter, the typical configuration is the long lever on the left (thumb side), and the short lever on the right (trigger finger side).
If you're a left hand shooter, the configuration is usually the opposite: the long lever on the right (thumb side for a left hand shooter), and short lever on the left (trigger finger side for a left hand shooter).
As typical of user interface controls, there are always compromises. What works for one person doesn't necessarily make it a
good choice for another.
The advantage of a long lever is it's easier to rotate due to the mechanical advantage offered by its length. A long lever on the trigger finger side interferes with the trigger finger, the tip of the long longer comes in contact with the top of the user's trigger finger. Some are used to it, some find it annoying. A short lever on the trigger finger side mitigates the interference, but the abbreviated length reduces mechanical advantage and requires more force to rotate. Though ASF's short lever is wider than the long lever (to compensate for the surface area lost from its short length), it still requires more force to rotate. This isn't an issue for most users, as most of us train shooting from our weak side less than 20% of the time and don't use the trigger finger side often, and users can always use the trigger finger to rotate the long lever on the trigger finger side.
As with most things in user interface controls, there is always some give and take. What works for one person doesn't necessarily make it a good choice for another. To find out if a long lever on the trigger finger side would bother you, you can pick up the lower with your left hand and rotate the safety to SEMI/FIRE. With a conventional 90 degree safety, the lever's tip points down at 6 o'clock. With a 50 degree or similar short throw safety, the interference is much less noticeable. If you don't like your trigger finger coming in contact with the tip of the safety lever, a long lever on the thumb side, and a short lever on the trigger finger side is probably the configuration that best suits you.