According to WABetaInfo, which tests the latest WhatsApp features before anyone else, the upcoming update will add a companion mode to your account that will allow you to connect another mobile device. This will be an enhanced version of the multi-device mode available and will be great for those who use both work and personal devices at the same time.
WhatsApp companion mode
WABetaInfo stated that this feature came with the update 2.22.15.13 in the beta version of the WhatsApp Android application. It looks like WhatsApp will show a warning that syncing is in progress while chat history is being transferred to all connected devices.
When this feature comes, users will have their chats synced between both connected devices as soon as they log in to WhatsApp, while switching, chats will be migrated from the first phone and automatically signed out of the current account.
This feature will remove the limitations in multi-device mode. As you know, you are limited to one smartphone and three other devices including tablet, PC, laptops.