Yes, that's a known limitation with line charts, two dimensions and multiple expressions.
The only way I know around this is trying to create ONE dimension out of the two, e.g. using concatenation.
I remember vaguely a post by John Wihterspoon that also tried to simulate the visual appearance of actually having two dimensions, by only showing the first dimension once per segment or something like this.
But unfortunately, I wasn't able to found this post in the forum tonight...