Until now it has not been possible to use the same Whatsapp account on two smartphones. Things are about to change, as stated on WaBetaInfo. This feature will be released little by little on Android beta version 2.22.21.6. This is a particular version of the ‘Companion’ mode, currently in the testing phase, which will also allow you to link your existing WhatsApp account to an Android tablet.
In order to understand if it is already possible to link your existing WhatsApp account to a secondary mobile phone, simply view the options within the registration screen to see if the ‘Link device’ option is present. If this option is present, you can finally use WhatsApp on another mobile phone.
After connecting your WhatsApp account to a new mobile phone, your chat history will be synchronized across all connected devices. As this is a beta version, some features may not be available yet.
Actually you can connect up to 4 devices at the same time. And personal messages and calls are always end-to-end encrypted even if WhatsApp is used from a connected mobile phone: when someone sends a message, it is sent to all connected devices, so the encryption is always preserved.
This feature is expected to be available on all phones in the coming months.