Well, Billy Williams (I think that's his name) didn't played Two-Face, Tommy lee Jones did, the twoface character is more important,so it doesn't really matter who played him. And from Batman 89 to Batman & Robin (4 movies) 3 BATMANS! so It woudn't be a problem to have 2 Harveys.
And remember this is the same show we're talking about! What if in BTAS, Batman appears in 1 episode from nothing with the JL suit?