If I get what you are asking you would have to jailbreak the iPhone then you would be able to use the wifi on the phone to share the connection with the iPad to get on the internet. If I get what you are asking..... Correct me if this is not what you are trying to do.
I've installed the trial but the ondemand part is not part of the trial so I cannot test that feature. willing to buy if it what I need works using MyWi.