In synchronous mode, both boards are treated as a single cluster, which is a great way to improve your machine performance if both boards are greatly varying placement contents (as is often the case with top/bottom PCBs). In asynchronous mode, the machine populates one board while the other one is moved into the placement area. This reduces your transport time, which can make a real difference for boards with short cycle times.
If you need to process very large boards, you can easily switch the conveyor back to single-lane mode.
As you can see, the flexible dual-lane conveyor provides lots of ways to get the best possible performance out of your SMT line.