If you want open mxd in new process you can use
import subprocess
subprocess.Popen(["C:/Program Files (x86)/ArcGIS/Desktop10.3/bin/ArcMap.exe", 'D:/temp/test.mxd']
for a syncronous process
import subprocess
subprocess.Call(["C:/Program Files (x86)/ArcGIS/Desktop10.3/bin/ArcMap.exe", 'D:/temp/test.mxd']
But you need close the new before the first.
Don't use
os.system("TASKKILL /F /IM ArcMap.exe")
because it close all ArcMap process windows
but you can use
import os
import signal
os.kill(os.getpid(), signal.SIGTERM)
That create a new windows... But that do the work