Is there any way to open multiple files with subprocess in one process?

Question

I am a Python novice, I cannot find an answer.

My test code is:

import subprocess
import time
#
subprocess.Popen(["C:\SAOImageDS9\ds9.exe", "./test1.fits"])
time.sleep(5.0)
subprocess.Popen(["C:\SAOImageDS9\ds9.exe", "./test2.fits"])

If the case of Notepad, I want to open test.txt and test2.txt in one Notepad window.

I run this code, but 2 windows were opened. I want to see files in one process.

Thank you.