Hackers can access your WhatsApp data by various means like via WhatsApp web or registering your number on another device. WhatsApp cannot work on two phones at the same time but hackers if register your number on another device, can easily get hold of all your chats including the personal ones.Hackers can quickly scan your WhatsApp QR code and access your WhatsApp conversation from anywhere in the world. However, this requires hackers to have a physical access of your phone to access the QR code.