According to a screen spotted by site WABetaInfo in a beta version of WhatsApp, the app appears to be working on a feature that would allow a user to chat simultaneously on multiple phones or with the same account on a phone and tablet. The screen shows instructions to register the device you are using as a “companion” by scanning a code with your main phone, but there is currently no code to scan.
A screen found in the previous beta version showed that devices could have the ability to sync recent messages even if they were end-to-end encrypted. Along with the “Register Device as Companion” screen that notifies users how to use WhatsApp on another device, this screen seems to support the work in progress.
Both screens were found in the Android version of the app. At the moment, it is unclear whether this feature will be available on iOS if it does, but looking at past examples, it can be predicted that iOS will not be left behind.
WhatsApp technically supports the use of your account on multiple devices through the Connected Devices feature, which currently only supports using computers as secondary devices. In other words, there should be no problem preventing the use of this feature on both iOS and Android.
WhatsApp rolled out Connected Devices as a public beta in November 2021, adding more enhancements thereafter. But while Connected Devices is useful for desktop users, it may not make much sense for those with secondary smartphones or tablets.