They have to share the same Order Id (Text - String). This method will also make share functions though, so you might want to use "Unit - A unit issued an order" event to detect which ability was casted.
I'm remaking "Espers". I need to have shared cooldown in order to make some kind of new "game-play" that is something different, new and original for the campaign as well as in the whole Warcaft 3.
There are five abilities, they all must have same cooldown(shared cooldown). The cooldown is short, and meant for the player unable to spam abilities(otherwise player would just spam all his abilities at the beginning of each fight killing normal enemies too quickly). The espers will have always 1000 maximum mana and huge mana regeneration forcing the player to use Espers' abilities frequently, but tactically.
How about starting a timer when you use an esper. Disable the other esper abilities for the player so the icons disappear, and the remaining icon shows the cooldown. When the timer expires, enable the disabled espers.
Or same thing but instead of disabling the abilities, use a boolean to check whether the abilities should be on cooldown. When an esper is issued to be cast, check the boolean and order the unit to stop if they are on cooldown.
you've just gotta copy both of the chain lightnings from the same original chain lightning, but they do still need the same ;Text - Order String - Use/Turn On: this will make it so when one chain lightning is used the other needs to cooldown even though it still hasnt been used