I've collected all the cards so far, besides the new Oathkeeper, and have come to a rules question I can't seem to find an answer too.

What is he official ruling on having two different heroes for the same base unit? Can I now upgrade two Paratroopers and two Stormtroopers?

From what I know from playing this game I would say that you just have to pick one, but I haven't seen it written out anywhere.