Say you have an ensign tactician and you train him in Beam: Fire At Will I to increase beam power. You give him 10 points in it.
Then when he becomes a LT, you train him in Beam: Fire At Will II. Can you use both I and II at the same time, or does II overwrite the first skill? Is it a case where the only way to get the more powerful skill Fire at Will II is to start with the I and then the first skill is lost, or do both remain?
The second part of the question may be debatable which is is it better to have a variety non duplicate skills or a smaller number of more powerful level II skills?