Key problems/questions.
 

I have 1 key that works on my X5 but the battery is dead in that key.
I bought 2 key's from ebay and had them cut to match the OEM key.
I can open the doors with them and turn the ignition but I can not start the car.
Is it possible to program a key from another BMW so it will work on my X5?
I have a key from my old E46 BMW that is the same key as my X5 key, but can it be done?

Sorry Hamar, but you fell for the ole Ebay key ripoff, the keys can be cut to open the doors, but a real BMW key has a chip in it to start to the car....sorry. Do a search and you will find more info about this subject.

There are programmers available that will allow this but the cost and the source of the software/hardware always put me off of purchasing one. So, unless you don't mind those factors, you are stuck with $tealership keys/prices and only if your vehicle has not had the maximum available.
You can try search on the web or fleabay for the programmers.
GL.