my only thought is that there could be some sort of physical 'stop' to prevent the mid pto lever from being engaged if the rear is already engaged. that shouldn't be impossible, or too much to ask.
Only on the 4x10 series. The 4x00 is levers-I helped WayneH add the Mid-PTO to his 4400 tractor, you really want to pull up the mid-PTO lever BEFORE engaging the rear!