If you have a newish Samsung, you can use Knox to maintain two independent WhatsApps on two numbers with only one sim in the phone.
Install and run the "secure folder" app. This is actually more than a folder - its a phone-within-a-phone. It has its own independent storage, app list, accounts, contacts, notes, calendar, permissions, notifications, gallery, browser, browser history, etc. It also runs on an encrypted partition.
Within Knox, login using a new / different gmail account. Install WhatsApp and go through the process of setting up a new account using a diff number on an old burner / temp phone. Deny permission to inspect the sim and instead say you will manually enter the verification code from SMS. Then the burner phone will get a SMS and enter that into the WhatsApp in Knox. You only need to make this link once.
That's it. You now have two WhatsApps running on one phone, each with its own number. You can switch off and stash the old burner / temp phone.
Inside Knox (Secure Folder) you can set notification permissions so that they don't show up on the main phone - only when inside Knox. Even if you set them to appear, it will show not details other than the app name.
You can rename "Secure Folder" to anything you like and even change its icon. Samsung Knox is quite wonderful.