I am not sure if Timo is right about not using different wire size, I know you can buy bi-wire from a few different manufactures and get a lighter gauge wire for the tweeter.
My advice however is to consider another wire over the Blue Heaven, at least to try, I had some in my system for over a year and could not get the right sound until I got rid of all of it. It really is thin sounding thru the mids IMO.
Try moving up to a single higher quality wire and not bi wiring, this seems to be a good strategy to improve the sound, often for about the same price as 2 runs of cheaper wire.
hope this helps