Just as a slightly different approach, gokubbs' advice is good, but you could just add cards from the second set that look good or you want to play. Then play 2-3 games and remove the cards that didn't work and replace with those that did. In this way you'll build up some knowledge as to what works and what doesn't and learn why gokubbs' advice was given.

I do generally agree that your best approach to making full decks out of two starters is simply to use two each of the 15 that best support the approach to play you want to try. Remember that Neutral can be in either Hero or Villain and since the Starter cards are all Blue, Red or Grey you have no cards you cannot slot at all (because your charatcers are Blue and Red and Greys can go anywhere).