Driving a single valve by two independent AC sources directly is a really bad design. If both sources are active (manual and OS) it may cause a short circuit with 50% probability. I highly recommend to avoid this setup.
To separate the two systems you should use one relay per station. This will atuomatically solve your LED problem as well. You need 24V AC relays with NO/NC/COM contacts.
See this picture as a guide.
Device=valve, V1 sine, V2 sine are the station outputs of the two controller.
Dual contact is not necessary in 24V AC setup The common wire can be directly connected of the two systems.