Restore WhatsApp backup from other phone (two phones, two phone numbers, same Google account)

Question

I have a complicated backup situation. An Android phone A with a (defunct) SIM card of phone number 1, logged in to Google as Google user G, has active WhatsApp chats. On a new Android phone B, with a spare SIM card using a different phone number 2, but logged into the same Google account G, also has active WhatsApp chats.

Giving up phone A and phone number 2, to use phone B with phone number 1 (using a new SIM card from the same provider), the desired goal would be to merge both WhatsApp chats into a unified WhatsApp chat history for phone number 1 on phone B.

As I already researched that it is not possible to merge two different WhatsApp accounts, we want at least to secure, backup and restore the chat history, including all media (photos and videos) from the recent years from phone A, phone number 1, to phone B, phone number 1.

As the WhatsApp migration guide does not cover this exact case with two different phones and two different numbers but using the same Google account, I am reluctant to put the new SIM card into phone B, fearing to lose the desired chat history and media, either getting the wrong backup (from phone number 2 chats) or no backup at all (as it happened with another replacement device despite having backed up to Google Drive).

Will it help to uninstall WhatsApp to make sure the latest backup is guaranteed to come from the right source? Thus:

1. uninstall WhatsApp from phone B (deliberately giving up phone number 2 chat history),
2. turn off phone B,
3. explicitly turn on "back up to Google drive" feature on phone A
4. explicitly "sync" accounts in Android settings
5. uninstall WhatsApp on phone A
6. turn off phone A
7. put the new SIM card for phone number 1 into phone B
8. turn on phone B
9. reinstall WhatsApp on phone B,
10. verify phone number 1 for WhatsApp on phone B,
11. choose to restore WhatsApp backup from Google Drive onto phone B. 12ff. Only after everything works, put phone number 2 SIM card into phone A, turn on phone A again, reinstall WhatsApp and verify for phone number 2.

Is this the correct way to go?

Answer 1

I can now confirm from personal experience, that the steps described above have lead to successfully restoring two distinct WhatsApp backups on both phones respectively, using only the full backups stored in Google drive, without having to use manually exported chat history files.

After a further examination of settings and system notifications, also on another phone where a similar backup attempt had failed, I see the important difference of both cases is to make sure that backups are actually created and saved. To do so, I had to allow WhatsApp to create backups and access Google Drive and make that there was enough storage space on the phone and that the phone is either connected to an unmetered network connection (like WiFi) or has a mobile data plan and has settings to allow to upload large files using the mobile connection.