....officially have only one plan for two devices. However, with a wifi connection a VOIP like Skype or Vonage would allow you to avoid paying for a monthly cellular phone #. You could pay for a wifi connection on the iPad. Let us know what happens.
Was looking to buy refurbished ipad with wifi and cellular. Already an ATT customer, so if I sign up for cellular with them for my ipad will that also cover iphone, or will I need different cellular plan for each