When you open a DB, it acquire lock over the data directory. Hence, you cannot open the multiple instances of same underlying DB. There is an option BypassLockGuard badger/options.go at baadc01dca6e349820ed63fb444ea14fd87cb8b5 · dgraph-io/badger · GitHub that allows skipping taking that lock.
This option is not recommended and should only be used when opening all the instances in ReadOnly mode.