The following is an example of a script that launches multiple JMAG-Designer instances.
※pywin32 must be installed. 

import win32com.client

import time

# Get the InstanceManager object of designerstarter

app = win32com.client.Dispatch("designerstarter.Instancemanager.241")

# Get the object of the specified key and launch JMAG-Designer.

designer1 = app.GetNamedinstance(1)

designer2 = app.GetNamedinstance(2)

# Display JMAG-Designer

designer1.Show()

# Load the model into first JMAG-Designer

designer1.Load(r"D:\2D_PM_motor.jproj")

# Display JMAG-Designer

designer2.Show()

# Importing the model into the second JMAG-Designer

designer2.Load(r"D:\3D_PM_motor.jproj")

# Wait 10 seconds

time.sleep(10)

# Closing the first JMAG-Designer

designer1.Quit()

# Terminate the object of the specified key

app.CloseInstance(1)

# Closing the second JMAG-Designer

designer2.Quit()

# Terminate the object of the specified key

app.CloseInstance(2)