It is possible for someone to see your WhatsApp messages from another phone if they are using a spying or monitoring app. These apps can be installed on a person's phone without their knowledge, and they can be used to secretly view all of the WhatsApp messages that are sent and received on the target phone. If you suspect that someone may be spying on your WhatsApp messages, you can check for signs of these apps by looking for unusual activity on your phone, such as increased battery usage or data usage.