How to implement filtered replication in couchdb by user with common documents across multiple users?

i have a server couch database with data of all users. I would like to replicate data of a particular user from this to another database (couch/pouch) on the client. i can't use different databases for different users on the server because there are documents that could be accessed by multiple users

eg: main database has

{"_id": "user1", "docs": ["doc1", "doc2"]}, 
{"_id": "user2", "docs": ["doc2", "doc3"]},
{"_id": "doc1", "name": "cow"},
{"_id": "doc2", "name": "monkey"},
{"_id": "doc3", "name": "dog"}

database for user1 should have after replication

{"_id": "user1", "docs": ["doc1", "doc2"]}, 
{"_id": "doc1", "name": "cow"},
{"_id": "doc2", "name": "monkey"},

and database for user2 should have after replication

{"_id": "user2", "docs": ["doc2", "doc3"]},
{"_id": "doc2", "name": "monkey"},
{"_id": "doc3", "name": "dog"}

i can get the user docs this way:

function(doc, req){
    if (doc._id != req.query.userid){
        return false;
    }
    return true;
}

but i can't think of a way to get the documents for a particular user using sorted ids since doc2 is common to both users and the documents doc1, doc2 and doc3 don't have user information.