Now I'm splitting offers by seller_id trying to make use of the UNION feature. But the original relationship key is sku_id + seller_id. So each sku and each offer has this key. I've tried to add sellers inside the $offers filter, but the result is very close:

{ "searches": [{
"collection": "skus",
"query_by": "sku_name,product_name",
"q": "*",
"per_page": 10,
"include_fields": "product_id,sku_id,seller_id,sku_name,product_name",
"filter_by": "$offers(offer_active:=true && clusterId:=['null','abd-1234'] && paymentFormId:!='null' && seller_id:=['670972463814f3334d650c74','66a9981f9e6d8686ba64a3f3']) && seller_id:=['670972463814f3334d650c74','66a9981f9e6d8686ba64a3f3']"
}]}
"search_time_ms": 370