I needed to do kind of the same thing as @vad32m did here.
thanks to him I could do it 
.
now, here is the problem, AFAIK we assign the range to the whole chart. but I think it makes more sense to assign it to the series. so then we can have for example range 0 to 100 for series1 and 5 to 10 for series2.
so what do you think?
Take a look at the pulse oximeter demo here.
I have 2 series and each y -primary and secondary y of the chart- is for one of them. one of the series is red and the other one is blue. I wanted to have the same color as the series for each y but it is not possible right now.
So I decided to have 2 transparent charts which are overlaid each other.
It seems that my needs are met by doing this.