Iphone and ipad on same apple id

Normally it is preferable to use the same Apple ID for all devices belonging to a single person. You would use each device separately but can choose whether (or not) selected things such as Photos, Contacts, Notes, Calendars, etc. are synched via the options at Settings > [username] > iCloud.

When using the same Apple ID on more than one device, several options are connected and synced and allow you to do different things.

For example, using the same Apple ID for both devices, would allow you to send and receive messages and calls on the iPad through the iPhone.

See: click here-> Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

It is highly recommended to use the same Apple ID on all the devices you own, unless there is a specific reason you do not want to do so.