@couchbwiss first, there is no couchdb bucket (only couchbase). Now onto your questions:
1 - N1QL is only available on couchbase buckets, not on memcache buckets. You can join between couchbase buckets though.
2 - You can store JSON in memcache buckets, but you will only be able to retreive it by its unique ID.
@couchbwiss if you want to join across buckets without N1QL, you need to open both buckets in the SDK, load the documents and then join them at the application layer. N1QL is the only built-in way to join documents across buckets without the application being involved in the steps.