Can two phones with different WhatsApp numbers use the same Google Drive backup?
Yes, and the backups will happily coexist.
My old phone had WhatsApp backing up daily to Google Drive. On the arrival of a new phone, I transferred the SIM into it, and activated a fresh SIM (with an as-yet unused number) in the old phone.
WhatsApp was installed on the new phone over Wi-Fi, and after going through the initial setup, restored the earlier chat backup from Google Drive. You can see the backup files online if you browse in Google Drive to Storage > Backups. They have the form WhatsApp #telephone-number# backup
In the old phone, WhatsApp detected the phone number was no longer valid. Having validated the new number, and going to Settings > Chats and clicking Back up, that created a new backup file in the list in Google Drive, which included the new telephone number.