Is it possible to sell an iPhone version and an iPad version of the same application or the app has to adapt to all devices?

I'm not sure about the actual requirements, but in my opinion you should create one app that adapts to all devices. Think about users who have both iPads and iPhones (and there are a lot of them). If you were one of them, you would appreciate being able to purchase the app once and simply install it on both your devices, right?

I think app review might push back if the only difference between the two is screen support, which you should be able to adopt in one app.

One Universal app can also facilitate less maintenance uploading and later when updating, etc. as well.

I would really suggest you make it Universal, especially if it's only for screen ratio reasons you are thinking about it.

As KMT said, updates will be twice the work.
Not to mention, it will split your download numbers, which might prevent your app from getting up the charts.

And once you make them separate, there isn't an easy way to combine the two user bases afterwards.

I made mine separate, for what I thought was a valid reason, and I have regretted it ever since.

Noel

I think there is not much benefit to doing two apps. Your iPad application needs to have support for live view and split view. So unless you want to drop suport for multitasking, you will need support for smaller screns in the iPad version anyways and it makes the iPhone one obsolete I think...