Be aware that if you copy/paste or drag/drop like this, then the associated metadata will also be updated and no longer be historically accurate. For example, action added and modified date attributes will be updated to the current date.
This means that if you’re OCD like me it’ll drive you batty. Additionally, if you use those attributes like I do [e.g. to track when projects were dropped], or average action life, then those calculations will no longer be possible because the metadata will be inaccurate.
Here’s how I recently merged two archives together. My current archive, and an older one from about 3 years ago. After the merge I have a little more than 5 years of history now. Your mileage may vary so make backups of everything before you attempt what I do below. Also note that I did all of this with OF 1. Finally, be prepared for several minutes of beachball fun, but bear through it.
How to merge two Archives
Open your second archive file in OmniFocus. Show all, and set all criteria to any status so that everything is visible.
a. Export an OmniFocus Backup of this [File->Export->Backup Document (Omnifocus)]
Open your current primary/master OmniFocus database. Create a database backup [File->Back up Database].
a. You will need this to restore your current data at the end of this process.
While in your primary OF database, restore the database from step 1 [File->Revert to Database Backup…]
a. You’re second archive should now be loaded as your primary database.
Archive your old data [File->Move Old Data to Archive]
a. Choose an appropriate date (e.g. I chose today since I wanted everything from the old archive)
b. After a lot of beachball this will finish and your primary Archive should now contain a merge of your primary and second archives.
Confirm Archive integrity by opening it and checking over data
a. Archive may be painfully slow due to transactions.
b. Compact your Archive database [File->Compact Database]
c. This will create a single compressed XML file of your data
i. My Archive of about 36,000 actions takes ~60 seconds to open in OF 1 after compact
Close all those windows and restore your primary database from the backup in step 2.
After all said and done, your primary archive should now be merged with the second.