You can now use Whatsapp on other device without an active internet connection after linked.
The WhatsApp beta for Android 184.108.40.206 offers the Companion mode to selected beta testers, according to the highly reliable source WABetaInfo. Currently, using the feature just requires you to scan a QR code in order to connect your WhatsApp account to an Android tablet. Though, you cannot currently connect your account to a second smartphone for now.
(image source: WABetaInfo)
The Companion mode is currently being developed, therefore certain functions might not function properly. According to WABetaInfo, you might not be able to view live locations and communities on a companion device, but accessibility will improve once the feature begins to roll out into stable channels.
It's important to note that Companion mode already supports cross-platform linking. This implies that an Android tablet can be connected to an iPhone's WhatsApp account. Even if the iPhone isn't connected to the internet after sync, the tablet can still use the WhatsApp account (same thing goes when the smartphone is Android-based).