Sorry I misunderstood, I thought you meant both horizontal. let me work on that, it shouldn't be hard to fixNo, if one control arm in completely horizontal the lines should still cross. They would only not cross if both control arms are horizontal which would not happen.
IN this formula, if either control arm is horizontal it gives an error, so something seems wrong.
In your diagram, the lower arm is completely horizontal and all the lines cross and the roll center is not zero.