To answer this question, it is important to understand the difference between an iMessage and an SMS text message.
For iMessages to appear on both your iPhone and iPad, both devices need to be setup with the same Apple ID in the Messages settings.
SMS text messages will not automatically appear on your iPad. You need to setup the Text Message Forwarding feature on the iPhone to have SMS text messages forwarded to your iPad.
This article provides additional information to help resolve the issue.
Prior to iOS 8.1, you could not receive regular SMS text messages on an iPad. Starting in iOS 8.1, the Text Message Forwarding function on the iPhone will allow SMS text mesages to be sent and received on the iPad. This article provides additional information about sending and receiving SMS text messages on an iPad.
iMessages are basically text messages sent from one Apple device to another Apple device. The message is sent through Apple’s servers rather than using the cellular provider’s network. If you are interested, this article provides more information on the difference between an iMessage and an SMS text message.
Posted in: Text Messages & iMessages