Rider and mount: But both are PC

HotW especially allows both Rider and Mount to be PCs.

What's the changes from these lines?

If a rider PC rides along with mount PC or similar non-minion intelligent creature, roll both their initiatives and use the lower of the two results. The two PCs act in either order on the same initiative count. While traveling in this way, the PCs each gain two actions at the start of their turns, instead of three, since the mount PC spends one action keeping the smaller one balanced on their back, and the rider PC spends one action maintaining their grip.

Your mount acts on your initiative. You must use the Command an Animal action (page 242) to get your mount to spend its actions.

You and your mount fight as a unit. Consequently, you share a multiple attack penalty.

You are in an attacker's reach or range if any square of your mount is within reach or range. Because your mount is larger than you and you share its space, you have lesser cover against attacks targeting you when you're mounted if the mount would be in the way (as determined by the GM).

Because you can't move your body as freely while you're riding a mount, you take a –2 circumstance penalty to Reflex saves while mounted. Additionally, the only move action you can use is the Mount action to dismount.