I have plenty of 3 or more player boardgames, but I would be bummed if I could never play this game with more than one other person. I was thinking perhaps of splitting each side into an Imperial Army player and an Astartes player, but I'm not sure how this will work. Any thoughts? Does anyone know of a way to meaningfully involve more players in this game?

I don't think splitting the forces that way would actually work within the rules. If I was in a situation where more than two people wanted to play, especially inexperienced players, I would just have multiple people on each side. This way you have a "war council" which makes decisions about how to spend initiative, what combat cards to play, etc. It isn't exactly like everyone has a separate faction or group that they control, but at least everyone is involved and has a say in what happens.