Yup, for now you can only attach a single template to a StackSet -- when you check a new template in, it will being pushing out to the accounts and only stop of you cross your fault tolerance threshold.
The most common approach I've seen with the current tools available is CodePipeline chaining -- trigger the pipeline in DEVO, when it completes successfully use a custom action to copy the template into a bucket in the next account, similar to this model: https://aws.amazon.com/blogs/devops/building-a-cross-regioncross-account-code-deployment-solution-on-aws/
I've had several customers ask for more complex deployment patterns for StackSets -- what would you prefer here?
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