Currently, we can use WhatsApp on up to four companion devices at one time (but we can only have one phone connected to our WhatsApp account at one time).

According to WABetaInfo, things will change soon...

WhatsApp is working on a function that will allow you to link a secondary mobile device to your WhatsApp account without requiring an active Internet connection on the main device in order to send messages. The chats will automatically sync across both devices.

The "companion mode" function was discovered in the new WhatsApp beta for Android 2.22.15.13 update.

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