Hey @johanna,
Ottoman.js does not support executing JOIN queries, N1QL itself of course does (as you have demonstrated). You’re best bet would be to perform these kinds of queries externally from Ottoman.js, and then if you need to, you can instantiate Ottoman object instances based on the data you have queries.
Out of separate curiosity, can you explain a little bit about your use-case for performing the joins?
Cheers, Brett