Merging two vaults into One

IF (and only if!) those vaults have the same structure (object types, classes, properties etc) you might be able to use replication to copy objects including documents from one vault into the other. It is important that you thoroughly check and compare all structure elements and in particular that you ensure that all elements have aliases and that those aliases are exactly the same in both vaults before you attempt to replicate any objects.

If there are minor differences in structure you can probably add the missing parts in the destination vault. 

If there are major differences in structure you may not be able to migrate everything or at least you may end with a rather messy result!

Make a copy of your destination vault and test migration on that copy before you attempt to do anything on the real destination vault!

Thank you very much for the reply, will double-check and ensure all elements line up, so can I simultaneously copy the data from both vaults into the new vault in parallel or should I first copy data from the old vaults  and into the new vault one by one ?

I would do this in at least 3 steps after you have aligned the structures:

1. Replicate structure elements from the vault that has a complete structure, or perhaps from both vaults if both of them have some parts that needs to be included in the new vault. Be very careful and check the results before you continue!
2. Replicate non-document objects and value list contents from both vaults, one at a time. Check results!
   If some of these objects exist in both of the old vaults you may end up with duplicates. This can get rather messy!
3. Replicate documents from both vaults, one at a time. Check results!
   Recreating relations to other objects may fail if the system cannot identify the correct objects after the merger. I cannot promise a success! 

Make sure you install any add-ons and copy the configurations from the old vaults. The latter can be tricky if they are not exactly the same! You may need to configure things manually or to copy sections of the JSON code manually to merge configurations.

Make sure you activate Event Handlers and any connections to external sources (mailboxes, external folders, external databases etc.)

Additionally, if the vaults has some history behind them, some years, there is a good chance you have your Metadata Card Configurations (MDCC) all done with property ID's instead of Aliases. As export&import does not keep ID's, you will have to fix MDCC rules as well...