If they both have the same schema then a mysqldump on each databases probably use the --no-create-info and one of the databases you can use --no-data and use that for the schema.sql. From there you can create a new database with all of the info from the dumps. Long as schema is the same it shouldn't be too bad. Of course make sure to backup beforehand, just in case.
https://dev.mysql.com/doc/refman/8.0/en/mysqldump.html
If the schema is different than INSERT and SELECT can be used to merge data. Either way a million posts is quite a bit, but honestly it'd be pretty cool to have the old threads, there were some great ones. Also we had various sections which are not here now for example "Do For Love" perhaps those miscellaneous posts can be integrated into a current section perhaps.
Something I was going to suggest is perhaps one member conducts an interview with a well known members perhaps or other interesting people. I remember when Kadafi's mom came on the forum and took our questions.