This is the ramp design I use:
Outer ring of red is the actual ramp, this is where dwarves walk. Blue squares are where you carve ramps. In my forts, I have one quarter of this ring pr. level, channelling out the rest.
The ring of white inside of that is channelled space. This is simply a pit because I like pits.
The inner ring of red are fortifications.
The inner circle of white is a pillbox. I have up/down staircases going in a + inside of that, leading all the way up to the top of the ramp. My entrance is about 8 z-levels down from the top, which means that if the entrance is ever breached, my marksdwarves will have a hayday of machinegunning.
When completed, a level (with fully smoothed walls) looks something like this:
(The stairway in the picture is 2 tiles too wide by accident, but I followed through with it all fort, so it still works just fine.)
It would not be impossible to have 2 stairways going across from each other at the same time either.
However, this is not a design you should use if you want your fort to be efficient by any means. This design is big, clunky and has lots of wasted space when walking up it. It is only so big because I think it looks good, and I don't mind that it's a tad inefficient if it looks good.