Past answers on Stack Overflow suggest that the single threaded nature of RocksDB means the answer is no, but official FB documentation has pages like this http://rocksdb.org/blog/2021/05/27/rocksdb-secondary-cache.html where multiple instances seem to be sharing a cache.
1)Are these instances using the same key range?
Just FYI. one process can have multiple RocksDB instances, each instance can have multiple column-families. They do not share user-data. Internally, column-families in one instance share WAL (write ahead log), manifest (tracking db information), option file. DB instances in one process share the same Env, threadpool, timer, cache.