The point is that each side gets a turn only once per day. You don't get a turn on your turn and then another turn on your ally's turn. You can postpone your turn if you want to act at the same time as your ally, but that doesn't give anyone an "extra" turn.
No, but he makes a fair point; it may be advantageous in some situations to make your turn, then to see what the enemy does, and then have an ally who can move after the enemy goes.
On first look, it might seem that way, but there isn't any actual advantage in any situation.
Start with a 1-on-1 battle: A gets a turn, then Z gets a turn, then A, then Z. Each stack you own has exactly one opportunity to respond to enemy actions before that enemy's next turn.
Now A gets a bright idea and decides to split so as to increase flexibility. Turn order is A1, Z, A2. But that's exactly the same order as Z, A2, A1, Z, A2, A1... we're still just alternating. I haven't accomplished anything except made it so that half of my army can't do joint ops with the other half.
Well, there are differences, though.
For example, A1 attacks, Z moves, then A2 gets to move BEFORE Z gets to heal, or gets to pop any more units. This could be key in some (narrow) situations.
Usually, though, it'd probably be better to attack all at once anyway.