Hej @hezor
you are right, currently your given scenario is not possible. But we already have some feature issues on github that address this:
https://github.com/Graylog2/graylog2-server/issues/3475
Additional the feature to be able to search on multiple streams with some virtual or meta stream would solve your issue
https://github.com/Graylog2/graylog2-server/issues/3473
or at least give you an option how to solve this.
Please feel free to add your use case to the issues to give them more examples.