I dont think you can "double-dim" a construction line across a construction line. Only solid sketch lines and points. Try using the end points of the construction line you want to mirror dimension.
Can't be done in 2010 either. The other thing you can't do is put symmetric sketch relations on construction lines. Another solution: Select the line, un-check the box for "Construction" in the property manager, add the dim/relation, then make construction again.
If both lines are construction lines, convert one to an object line, make the dimension, then convert back to construction line.
Otherwise, just grab the line's endpoint.
think what you are asking it to do...it has no idea which one you want to be the driving one.....
i no you may not like it but thats how it is... do like all the others have suggested. thats how I have done it forever.
Without seeing a picture of what you want, I'm having trouble visualizing it.
But it sounds like what I do for getting the length of bend round bar in my BOM...
I dimension the centerlines of the two sides, then add an equation in SWX to add them together and assign that to a dimension that drives a diameter or length of a line, or simply just a variable for my BOM.