In reply to mick taylor:
Normally there is enough play in the top sliders that you can lift the opening door and then move the bottom out off the bottom slider. Some have blocks screwed into top of the frame near the slider to prevent you from being able to do this from outside when the door is closed. If so, open the door as far as it will go and then remove the blocks then close it slightly to allow you to remove the door.
Hopefully this will give you that extra space required. Some designs allow you to open and remove both sliding parts so you could repeat the process above. Other designs have one fixed pane.