Correct usage of unique() indexes on Titan : PermanentLockingException - multithreading

Question

Update based on later experiments :
Initially I assumed that the PermanentLockingException was due to my usage of unique indices with locking. I removed this unique() property from my index graph configuration and I still run into the same PermanentLockingException.
To clarify, I have two separate indices and I'm trying to update the two of them concurrently. I'm not trying to update the same index from two threads and running into this issue.
This indicates to me that regardless of the index type, some kind of a global lock is obtained by Titan for every index-based transaction ? This means that at any given time Titan can update only one index tree ?
Is this correct ?
Thanks
I'm using Titan 1.0.0 with Cassandra backend. I created two composite indexes on Titan :
index1 = mgmt.buildIndex(indexName, Vertex.class).
addKey(indexPropertyKey1).addKey(indexPropertyKey2).unique()
.buildCompositeIndex();
index2 = mgmt.buildIndex(indexName, Vertex.class).
addKey(indexPropertyKey1).addKey(indexPropertyKey3).unique()
.buildCompositeIndex();
My intent behind doing this is to ensure that there is exactly only 1 entry in the backend with Key1+Key2 composite combination. I dont want any duplicates. I want to enforce the same thing with index2.
My understanding was that these are two different index constructs so they should not run into any conflict between each other. But I notice that when I'm trying to do a concurrent access/update on index1 and index2, I get a locking.PermanentLockingException from Titan. If access them serially I don't run into any trouble.
Some questions I have :
Is my usage of unique() correct ?
I'm not explicitly setting ConsistencyModifier.LOCK, so I'm assuming this is getting set by default to true by some other layer ? Does it matter one way or the other ?
These are two different indexes. So their access should be mutually exclusive. Am I missing something here ?
They do share one of the property keys, key1, does that matter ?
How can I allow concurrent access of these two different indices ? Can I allow this explicitly ? What does it cost ?
Thanks
Here is the error in-case someone is interested :
2016-09-01T12:17:19.29-0700 [App/0] OUT [Z: C:] 2016-09-01 19:17:19 ERROR [pool-30-thread-1] c.t.t.g.d.StandardTitanGraph [StandardTitanGraph.java:695] Could not commit transaction [8] due to storage exception in system-commit
2016-09-01T12:17:19.29-0700 [App/0] OUT com.thinkaurelius.titan.core.TitanException: Could not execute operation due to backend exception
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.diskstorage.util.BackendOperation.execute(BackendOperation.java:44)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.diskstorage.keycolumnvalue.cache.CacheTransaction.persist(CacheTransaction.java:87)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.diskstorage.keycolumnvalue.cache.CacheTransaction.flushInternal(CacheTransaction.java:141)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.diskstorage.keycolumnvalue.cache.CacheTransaction.commit(CacheTransaction.java:198)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.diskstorage.BackendTransaction.commit(BackendTransaction.java:135)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.graphdb.database.StandardTitanGraph.commit(StandardTitanGraph.java:692)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.graphdb.transaction.StandardTitanTx.commit(StandardTitanTx.java:1352)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.thinkaurelius.titan.graphdb.tinkerpop.TitanBlueprintsGraph$GraphTransaction.doCommit(TitanBlueprintsGraph.java:263)
2016-09-01T12:17:19.29-0700 [App/0] OUT at org.apache.tinkerpop.gremlin.structure.util.AbstractTransaction.commit(AbstractTransaction.java:94)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.ge.predix.acs.privilege.management.dao.GraphGenericRepository.save(GraphGenericRepository.java:113)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.ge.predix.acs.privilege.management.dao.ResourceMigrationManager.doResourceMigration(ResourceMigrationManager.java:52)
2016-09-01T12:17:19.29-0700 [App/0] OUT at com.ge.predix.acs.privilege.management.dao.ResourceMigrationManager.run(ResourceMigrationManager.java:34)
2016-09-01T12:17:19.29-0700 [App/0] OUT at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
2016-09-01T12:17:19.29-0700 [App/0] OUT at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
2016-09-01T12:17:19.29-0700 [App/0] OUT at java.lang.Thread.run(Thread.java:745)
2016-09-01T12:17:19.29-0700 [App/0] OUT Caused by: com.thinkaurelius.titan.diskstorage.locking.PermanentLockingException: Expected value mismatch for KeyColumn [k=0x 16-165-160-114-116- 30- 97-116-116-114-105- 98-117-116-101-243, c=0x 0]: expected=[] vs actual=[120-133] (store=graphindex)