If you make type on a path in Illustrator on a circle, and you cut the circle in half 50:50 to get the type the right way up in each half - can you then adjust the half to make it eg 60:40 or some other proportion (to allow for changing text length in the top half)??? Or do you have to cut the circle in the exact place you need to each time??
Instead of cutting one circle, just create two circles on top of each other and use one to place text across the top and the other circle to place the text across the bottom.
You can then adjust how far you want the text to wrap around