not to be that guy but kelsier says in the book you can remain stationary in the air with just one anchor
You can, but you'd have to be directly over the anchor. It'd be more akin to balancing on the end of a long pole as opposed to having stable legs to rest on.
With multiple targets, on the other hand, you can gain elevation and maintain that elevation anywhere within a triangle formed by any 3 anchors within range.
It'd be the difference of trying to balance on a unicycle vs a 3-wheeled ATV.
If it is simultaneously doesn't that count as one action?
So if I stab 2 people simultaneously wouldn't it count as one action?
By default no. Why? Because you can't attack multiple targets with one action*. Nothing in the Multiple Targets stunt changes that. It does offer up more options for the player, gaining extra actions in a beat isn't one of them unless they get 3 Nudges on the first attack to catch a beat.
*Except Extras in Epic combat, and even then you don't declare multiple targets you just kill these guys quickly, whether or not you have the Multiple Targets stunt.