WhatsApp is working on an upcoming update that will allow you to chat on multiple devices such as phones and tablets with the same account. It registers your device as a “companion” that scans your code from the main device.
Encrypted messages may be able to sync, according to an earlier beta. A screenshot of this screen, together with the “Register Device as Companion” screen, shows that this feature is in development.
Apps for Android had both screens. This means that you will be able to use this feature on your second phone or tablet as well. It is unknown if this feature will be available on iOS, but it has been done before. A feature called “Linked Devices” lets you use the same WhatsApp account on more than one device at the same time. It currently only lets you use your computer as a second device. This feature can be used on both iOS and Android, and it works both ways.
After a public beta in November 2021, WhatsApp has upgraded Linked Devices (though there are still a few unsupported features, depending on what your main device is). In addition to PC users, Linked Devices doesn’t help people with additional smartphones or tablets.