I don't see anything implausible about a second shooter having Kennedy in his sights, waiting for the initial shots to be fired, and pulling the trigger once he heard the initial shot.
There are a lot of implausible aspects to all of this, but that... doesn't seem like one.
Either there are two shooters working together from the beginning, or one knows about the other and the other does not.
But that needs a bigger conspiracy than just kill JFK.
What about that seems implausible to you?
As for coordinating their fire, what about that seems implausible?
"I'll shoot first, once JFK reaches point XYZ. When you hear my shot that's your signal to shoot. Be ready and have your finger on the trigger, because obviously Secret Service is going to floor the gas and GFTO ASAP as soon as the first shot is fired"
That seems entirely within the realm of plans two dudes could come up with. I don't think it necessarily points to some larger conspiracy.
Much easier to keep everything quiet too. The difficulty of keeping secrets increases exponentially with the number of people who know it.