You can't. It's either low or high horn separately. Even on pc, if you press both, it's whatever got pressed a millisecond faster. I think some trains do have 2 tone horns like the ice but for other trains, you only have separate horns.
And if you look at most of the British rolling stock at least, the High and Low horns are on the same lever in the cab, just you pull in different directions for each (thus would be impossible to get both at the same time)
