That's actually the way it works now, the issue is that most games do not support separate Y axis sensitivity, so you're getting horizontal match for both axis. Which might be fine depending on your preference, but there's no way of doing vertical for both axis with the current implementation.
Splitting it into a horizontal and vertical option makes it work the same way monitor distance works, you choose one axis and the other one will match 1:1 (i.e. same 360 distance).