since the SHIFT button is "hardwired" and not mapped, i also went for it as change to deck c & d in my traktor 1.2.7 mapping. it seemed to be the most straightforward idea.
unfortunately it cannot be set to toggle, that is what you're missing, right?
in my new mapping for TSP2 (soon to be released) i use the CUP button as shift button (toggle) between track decks (a&b) and sample decks (c&d).