Answer
Business entities cannot be merged between backups/databases, however they can be split up. Splitting databases might be useful in cases where a solid chain of backing up has not been achieved between multiple user PCs.
To split entities into separate dbs/backups:
- Make a backup with both companies
- Delete company B
- Create backup with remaining company A
- Restore original (two company) backup
- Delete Company A
- Create backup with remaining company B