I had previously submitted a feature request to merge clients on https://forum.itflow.org/d/216-new-feature-merge-clients/4.
The suggested workaround was to hack the database changing one of the columns. I had the idea if I export the data from client b, I could go to client a and import.
This worked for contacts, locations, and networks. Assets, Licenses, passwords all errored with bad count. Export the sample csv with header, paste the data, still bad count. Fortunately there was little data and I was able to recreate the assets, licenses, and passwords by copy and paste.
Assets exported with a header 'Operating system' but expected 'OS' when importing.