I'm not completely sure why this question asks about two spirits. This wonderful story technically has three spirits that visit Scrooge. If you count Jacob Marley as a spirit, then the story has four spirits. The three spirits that visit Scrooge are similar in that they all exist to teach Scrooge something about himself and hope to change him into a better man. Beyond that, they are quite different in appearance and personality. The Ghost of Christmas Past is an odd combination of a small child and old man. The ghost is quiet, thoughtful, and quite compassionate. The Ghost of Christmas Present is described as a "jolly giant." He is perpetually happy, is loud, laughs a lot, and is a general pleasure to be around. This makes sense based on his message being about the present and Christmas time. It's a season of happiness and love. The Ghost of Christmas Yet to Come probably contrasts the most with the other two spirits. There is a very strong Grim Reaper vibe to him.
It was shrouded in a deep black garment, which concealed its head, its face, its form, and left nothing of it visible save one outstretched hand.
He's scary to look at, but the main difference is that he doesn't speak at all. There is no verbal teaching or reprimanding of Scrooge. This ghost does all of his teaching through showing Scrooge potential eventualities.