Hmmm... It looks to me like you should be able to as if you'll look under XY Graph Properties->Plots, each plot has a drop-down list of items specifying the Y-Scale and X-Scale that the current plot is using.
The only problem is, each list only contains a single item - either the current X-Scale or Y-Scale, and under the scales tab (unless I'm blind), I can't see any options that might allow you to create new scales... As a stop-gap, just write your own VI that will scale the second plot's Y values to correspond to the first plot's Y scale - should take you maybe 5 minutes, and then you can always take it out later if you find a new way. Sorry I'm not much more help than that.
