I had some trouble with split tools too. After trying the split tools. I installed split sausage separately, and have both donut and sausage under "Tools / split tools" submenu.
The thing about split sausage is somehow it does not break the curves. You can do the same thing as Cotty shows by drawing in the connecting top edges, softening those edges, then "recurve" (or "weld) the side curves with those plugins .
I just thought of something that probably bears mentioning. This method is probably OK for what you want to do, but we find that moving a curve this way changes its path in plan view. If that matters, you would need another way. If you turn on edge style to "color by axis" and hidden geometry, you will see the vertical (blue) edges on the path become non-vertical (black)-- so to be picky, it's not exactly how the ramp would be built, or how one would imagine the model to be made. But it's handy.
Another approach is not to start with the level ramp at all. For example "shape bender" plugin can fairly closely create the ramp you have with one curve and a small straight ramp with the width and the start and end elevations.
P
