The 08 C's big three were all Alphas. Heat's Lebron era big three were all Alphas. So you can have more than one Alpha on a team.
An Alpha is a guy you can go to and have a lot of confidence in. At this point Tatum and JB were probably never that for me. You always feared TOs with JB and clunked step backs with Tatum were coming too often. You really want to feel like your Alpha is a closer. You can also have non-scorers be Alphas, like a TD who was pretty much just a 20 point guy or Kidd who commanded the offense and could hit Fts.
An Alpha should make things happen and make positive plays. They should set a tone and dominate most their match ups. You shouldn't be able to play against one, one on one and feel comfortable you can stop them at the end of the game.