in megaton and in Tempenny tower
The houses are mutually exclusive, you can only ever have one at a time. To earn the Tenpenny tower house you must complete the quest where you blow up Megaton, thus destroying the megaton house.
And if you want to live in Megaton, you have to deactivate the bomb in Megaton making it impossible to get the Tenpenny tower house.
Details on the quest can be found @ The Power of the Atom