This is just an idea, I never done this. What if the bell valve is combine with a swivel valve for multiple grow beds or a large bed with multiple Bell Valve.
The idea is, synchronization of bell flush will occur due to swivel valve is the one that start the flush. This is made possible by making the High Water level of the swivel valve higher then the level for the Bell Valve.
Bell valve will not flush as water reaches its high level, just overflow, since the high level of the swivel is not reach yet.
Common manifold will be filled, but no flushing action yet. Once High level reaches on the swivel valve the flush started. This will cause the bell to assist swivel flush and continue till finish.
As I said its just an idea... I'll try this out one of this days. I am toying with this idea because if you have large grow beds, you can use more then one Bell valve to make the water flow out faster and where synchronizing the flush is required.