can krpc control multiple ships at the same time?
I try to control another rocket by Control.ActivateNextStage() when I control a flighting plane. Then it will throw an error.
Does it only control the Active vessel?

If someone knows how to do this with kRPC, I would love to know!

If someone knows how to do this with kRPC, I would love to know!
I do not read the source code.
But I confirmed after I experiment that it is could to done within 200 meters.
Just make a series of variables representing multiple ships.
But the rocket speed is too fast for the range of 200 meters so I do not know what it can do.

I believe you can since you can get a list of vessels. I suggest you use the mod Physics Range Extender to get rid of the 200m range limitation.

Yes multiple vessels can be controlled. But they must be within physics range of one another. This is a game limitation we can't avoid.