Multi index search, with kNN on one index

Elastic Stack Elasticsearch
1

I have two indices, one with users and one with user posts. I want to search both indices simultaneously, and rank them. As part of this search, i want to do a kNN search as well as a standard text search. However, the kNN search is only applied to the posts, not to the users. Is this possible? I have tried the following:

{
    "_source": ["_id", "video_url"],
    "track_total_hits": true,
    "query": {
        "exists":{
            "field": "avg_vector"
        }
    },
    "knn": {
        "field": "frame_vectors.frame_vector",
        "query_vector": [
            0.0159149169921875,
            -0.031982421875,
            0.058349609375,
           ....
        ],
        "num_candidates": 100,
        "filter": [
            {
                "exists": {
                    "field": "avg_vector"
                }
            },
            {
                "exists": {
                    "field": "frame_vectors"
                }
            }
        ]
    }
}

but unfortunately getting a list of posts (which match the kNN) and then the user index is erroring:

"_shards": {
        "total": 16,
        "successful": 15,
        "skipped": 0,
        "failed": 1,
        "failures": [
            {
                "shard": 0,
                "index": "user",
                "node": "RUSE4yhWTgufSSfBLUnVoA",
                "reason": {
                    "type": "query_shard_exception",
                    "reason": "failed to create query: [nested] failed to find nested object under path [frame_vectors]",
                    "index_uuid": "VCQz4HZaRsCBeeQlLFPATw",
                    "index": "user",
                    "caused_by": {
                        "type": "illegal_state_exception",
                        "reason": "[nested] failed to find nested object under path [frame_vectors]"
                    }
                }
            }
        ]
    },