I recently had a problem while transferring my WhatsApp. I had lost access to my primary phone (Samsung Galaxy A71) for a while and had to use a secondary phone(Huawei P20). During that time I installed WhatsApp on my secondary phone but I did not use the Google Drive backup to restore the chats and instead started to use it as is. Now that I have gotten my primary phone back I was wondering how do I merge my WhatsApp data between the two phones on my primary phone because it already has the most data and the local storage and secondly I have to return the secondary phone as soon as possible.
According to Google, there is some 3rd party software which will merge 2 WhatsApp backups. You will have to generate a backup file of both your installations.