Mowcius: the hardware would let you do that. My software, however, sends the user defined characters to both hd44780s. My philosophy has been to try to make the software for the 40x4 act like its one device. You'd have to change the API if you wanted to be able to tell the software which lines you were defining the characters for. I suppose you could write your own code outside LiquidCrystal (or subclass it!) to LOAD the user definitions and LiquidCrystal would never know the difference.
I knew you would have the answer 
Right. I might look into it. I think it might be useful for a few projects I have in mind.
Thanks,
Mowcius