Best answer
The trigger is per channel, so you need one trigger each, but the work after the trigger can be shared.
Why the trigger cannot be shared: an Instagram comment and a Messenger comment arrive as different events with different identifiers, and the actions available differ too. Public replies work on Instagram, and handing a conversation to the Page Inbox is a Messenger only thing. A single trigger would have to pretend those differences do not exist.
How to avoid maintaining two copies:
- Build the shared part once as its own flow. The questions, the branches, the tagging.
- Build a thin automation per channel: trigger, any channel specific first message, then a step that starts the shared flow.
- Edit the shared flow when the content changes. The two thin ones almost never change.
This also keeps your reporting honest. Two triggers means you can see that Instagram produced four times the volume, which one merged automation would have hidden.
One caution when you do this: the shared flow runs in the context of whichever channel started it, so any step that only exists on one channel needs a branch rather than an assumption. Public reply is the usual one people forget, because it silently does nothing on a channel that has no such concept.
The Build and publish an automation article covers calling one flow from another and what publishing each one separately means.