not that one cannot recite both, but just FYI.
"Namo" is a form of Reverence or homage to Amitabha Buddha. it can be recited more in a form of respect, formality for beginners, or in masses where there are mixed categorise. for those who had already established a deep realationship with Amitabha Buddha, one can no need be so formal just like an old friend, you can forsake the "politeness" per se.
there was a Patriach who introduce people to recite Namo Amituofo, but he himself recited Amituofo, and people ask him why. his answer was that his purpose is to recite to go to pureland, whereas the others did not have the aspiration yet.