unione dei dati di due account diversi

It won't be a simple operation. You will have to export all activities from the wrong account as  "Original" and then import them into the account you want to keep using (at the address https://connect.garmin.com/modern/import-data). The you can try exporting also the daily monitoring data on the Daily Summary page - for each day separately (i.e. https://connect.garmin.com/modern/daily-summary/2023-07-21), and again importing the in the right account. However, the import does not always work as one would expect - some of the metrics may stay unaffected anyway (that was at least my experience, the last time I tried it).