Hello

that's a very hard task, and I strongly advise you work on a separate (third) installation (copy of one of the two)

the main problem is the identifiers of the pictures and the album ('id' column on "piwigo_images", "piwigo_categories", "piwigo_groups", "piwigo_users" and fields in many other linked tables, and I certainly forget some primary tables)

my idea is to look for the higher id of one of the database and start reindexing the other one starting from this id
- you must ensure that all data in linked tables are consolidate
- once you finish don't forget to update the AUTO_INCREMENT property of tables
- also ensure that all physical path will be correct after migration

there is no automated tools to do this
very very hard task....