Is it possible to have two freeway on-ramps,(one going north and one going south), and have them both go to the same city?
Sure. There can be two cities, one at the Eastern edge and one at the Western edge of a loop formed by a freeway that runs in a circle. From the city on the West, the North ramp runs counter-clockwise and the South ramp leads clockwise to the city on the East.
There are several such loops near Houston. For example, from Jersey Village, 8N and 8S both go to Houston. 610 is similar.