I don't really understand what you're asking or what you wish to achieve by connecting them. If you mean sharing data than iCloud will allow you to automatically sync photos taken on each device, your calendars, contacts, reminders and many other things such as your Pages, Keynote etc documents.
As someone else mentioned, carrier dependant you can share your data connection using Personal Hotspot.
You can also have your iTunes purchased music synced up between the two, if you have lots or other music on a PC/Mac that you want cloud access too then I'd like into iTunes Match.
You can share pictures and videos over AirDrop and you can share data using Personal Hotspot. That's pretty much it for direct interconnectivity between the two devices.
With iOS 8, you can:
1. connect to your cellular connection on your iPhone automatically (with one tap).
2. Make and receive a call on your iPad.
3. Also SMS.
4. Continue working from your iPhone on your iPad (Apple apps).
With iOS 8, you can:
1. connect to your cellular connection on your iPhone automatically (with one tap).
2. Make and receive a call on your iPad.
3. Also SMS.
4. Continue working from your iPhone on your iPad (Apple apps).