well current wording is "Played on your opponent's side of the field. You can see your opponents hand as long as Spy lives."
this means that since the opponent now controls spy, then the ability You (opponent) can see your opponents (You) hand as long as spy lives. i think i see a problem
but i dont think negative attack would fit a spy, i mean it would have to fit in, why not just give them both 0 attack and be done with it