[VariousRen]

Having players play the NPCs can be an interesting learning experience for them, especially if the NPC is decently outmatched. PC's tend to play the numerically stronger character (higher skill, better weapons, more HP, ect), so they can feel a little lost when they're on the back foot and trying to figure out how to hit back effectively. I've found players surprisingly willing to play an NPC to the hilt against the party, because there's no social expectation that they will pull their punches like in-party PvP.

If you're not comfortable with that, I'd say run all the combats at the same time. There are a few benefits to this over sequential combat. First, there is plenty of time between turns to decide on your action, which helps newer players work out math on their attacks, read up on maneuvers they don't use much, and think of witty remarks. Second, all the players are engaged, there's no dead time for anyone. Third, players will be engaged with eachothers turn, and will pickup on how things are done that they can use. You only need one brave player to try a Deceptive Attack to get everyone using them, or Committed Attacks, or targeting hit locations.

As a split measure, you could divide up the combats into batches of 2 or 3 PCs fighting, the others watching in the crowd. The watchers can have conversations going with other NPCs, commenting on tactics, ect. "See there, such skill wasted on an easily defended blow. Surely he has more to show off here than that!".