You need to use both files to restore for “all the data set” in the cluster. But to restore one shard only with a backup in the past, using the oplog file for that shard is enough as shards are independent.
You need to use both files to restore for “all the data set” in the cluster. But to restore one shard only with a backup in the past, using the oplog file for that shard is enough as shards are independent.