import iterm2

async def main(connection):
    app = await iterm2.async_get_app(connection)
    window = app.current_terminal_window
    tab = window.tabs[0]
    print(f'Before: {tab.sessions}')
    await tab.sessions[0].async_split_pane()
    print(f'After: {tab.sessions}')

iterm2.run_until_complete(main)

# Output is:
Before: [<Session name=vim id=4058DAA1-B6C6-4376-B822-7A6E5EDADBB0>]
After: [<Session name=vim id=4058DAA1-B6C6-4376-B822-7A6E5EDADBB0>, <Session name=  id=A82F1674-C386-4CAA-868B-507C4474A1BB>]