I would like to do a project based on quadcopter.
The plan is to control multiple drones ( two copters for the sake of feasibility and affordability)
Main aim is to control two drones where these two copters will autonomously adiust their distance between them by sending one's GPS  coordinates and compasse orientation  to each other. And use these received information to keep safe distances between them.
*Also we require to keep one copter stationary and control the other one*
Is pixhawk a better choice for this purpose??

1. You can use nrf24l01 modules since your setup consists of only two UAVs. If you have multiple UAVs you can use MQTT supported modules.
2. However you may have to modify the flight controller firmware to suit your requirements.

You can also look into INAV flight controller source codes. Their source code is small compared to Pixhawk.
Pixhawk source code is also well documented to add an extra feature if you wish to do so.
As long as you can tweak the firmware the flight controller choice doesn't have much effect on it.

A typical unmanned aircraft is made of light composite materials to reduce weight and increase maneuverability. ... Drones are controlled by remote ground control systems (GSC) and also referred to as a ground cockpit. An unmanned aerial vehicle system has two parts, the drone itself and the control system


https://www.gangboard.com/microsoft/windows-azure-training
https://www.gangboard.com/rpa-online-training/uipath-online-training

Ardupilot already supports swarms and can be fitted to some cheap boards http://ardupilot.org/planner/docs/swarming.html