Don’t think of a cross-db transaction. This type of operations can be made configurable. For ex: Ecto.Multi.query(). When creating the query, it can be created by taking the necessary parameters from the multi-repos settings.
I think this is possible e.g. with something like Dblink in postgres. However to do this in Ecto we will have to use two different connections and Two different Repos, which is not supported as of now. However there could be multiple workarounds but something like this without any SQL extentions/plugins is not possible to my knowledge.