How to talk to two ROS devices over serial?

edit

ROS serial is designed to communicate between a ROS node and a simple peripheral device which isn't running ROS such as a micro-controller.

If you're trying to communicate between two ROS nodes running on two different raspberry Pis all you need to do is make sure that they are both on a shared network (Wi-Fi or ethernet) and then the distributed nature of ROS will handle everything for you automatically.

Is there any particular reason why you thought about using a serial connection for this?

Update

Okay I understand your task now. To be honest, you could do this with a much lower power device such as an arduino or teensy on the drone. A raspberry pi is overkill unless it's doing other work as well. I'll emphasise again ROSSerial doesn't connect ROS nodes together it connects peripheral devices to ROS.

Using an arduino you could follow the tutorials, the blink tutorial controls an LED which is one of your tasks. You should be able to extend this to control a servo as well.